The noise of the explosions was deafening, shaking the ground, shattering windows, setting off car alarms. Bullets and shrapnel flew out in flaming arcs, setting fire to part of a checkpoint and sent families with children scurrying for cover.
The blasts reverberated for 22 minutes, sending the local social network into a frenzy: Mariupol was once again under attack. Car-loads of fighters set off from Donetsk and Slovyansk to provide support. But there was no raid by forces of the Kiev government: an armoured personnel carrier it had abandoned had been set on fire; the massive blasts were the result of ammunition detonating inside.
It was, according to differing accounts, a deliberate decision by separatist militant commanders to destroy the carrier, sabotage by fifth-columnists or – the most likely explanation – the handiwork of drunken pickets who had been manning a freshly erected barricade made from the contents of City Hall, which had been gutted the previous evening.
The febrile atmosphere was, however, understandable. Apart from any attacks, a referendum due to be held today by pro-Russian separatists in a number of cities in eastern Ukraine, including Donetsk, aimed at triggering independence, was also stoking tensions.
Ukraine's acting president, Oleksander Turchynov, declared yesterday that a vote for independence would be "a step into the abyss for these regions". "Those who stand for self-rule don't understand that it would mean the complete destruction of the economy," he said.
Mariupol has been the focus of repeated attacks by government forces. The latest came on Friday, on one of the most revered anniversaries in the Russian-speaking half of the country: the commemoration of victory over Nazi Germany. An assortment took part in the assault, including a private army supposedly bankrolled by an oligarch – the "men in black". It left between seven and 20 people dead, depending on accounts, after fierce clashes centred on the main police headquarters.
The government claimed that its attack on the building, setting it on fire, came only after protesters had occupied it with the aim of taking over the armoury. Residents angrily disputed this, insisting that "fascists" from Kiev had tried to storm the building to get hold of the weapons, shooting at the police, because they knew the local force "would not turn against the people".
It was unclear just how many were killed in the clashes. I saw two bodies, one of a police officer and another a civilian with an armband of Ukrainian colours. This person was part of a group, according to residents, who had appeared in support of the troops. Two others were brought out of the building and, including reports from the hospitals, the number of deaths was likely to be around 10.
The country's excitable acting interior minister, Arsen Avakov, who keeps a fairly unreliable chronicle of military matters, wrote on his Facebook account: "A terrorist group of about 60 men armed with automatic weapons attacked the police headquarters … About 20 terrorists were destroyed and four taken prisoner. To those who come with weapons and who shoot … To them there can be only one answer from the Ukrainian state – annihilation."
Mr Avakov's postings are normally treated with scepticism but this one was seized on by some in Mariupol. "He is right about more being killed than the media is saying," maintained Yuri Koralinkov, a pro-Russian activist. "In fact, the number was 36, most of them in there," he said, pointing to the charred shell of the police headquarters. The front steps had been turned into a shrine of flowers and candles. Turning back from them, Ludmilla Vladislavia corrected: "No, it was 42. I heard it myself from the firemen. This is another Odessa."
Others were quick to take this up. It was a highly emotive comparison: up to 55 people had died there, separatist demonstrators, after being trapped in the city's trade union building by a nationalist crowd, which had then set it on fire with Molotov cocktails.
"This is now the junta's favoured form of murder, burning people to death. They call us terrorists, but it is they who want to terrorise.
"What happened at Odessa disgusted so many people, so many volunteered to join the people's militias. This will have the same effect with the deaths and injuries," Mr Koralinkov said.
At the city's main A&E department, Irina Pomarovna, the deputy director, said around a dozen wounded had been brought in; one of them, a 38-year-old policeman, had subsequently died.
"We got the ones brought in by people in their private cars. Another hospital had got around the same numbers," she said. "There were some burn victims, but also lots of penetrative wounds to chest and abdomen. We were expecting large numbers of casualties because of the rising numbers of attacks; staff dealing with trauma had been on stand-by, but this is still a shock."
There was another type of shock when a woman from City Hall came to collect the personal details of those brought in from the attacks; they had already been given out to two men, claiming to be officials. In Odessa, names and addresses of protesters had been leaked from hospitals and ended up in the hands of Ukrainian nationalists; threats and intimidation had followed.
Andre Petrovich, who had undergone surgery for gunshot wounds to his leg and hip, was not aware of the list that had been inadvertently released. But he was already deeply concerned about his safety. "There have been people arrested in hospital. Not all the police sympathise with the public, some are with Kiev. Also, you have these fascist militias. They, too, can come and snatch you," he said. "So you're vulnerable here."
A 38-year-old-builder had been at the city centre on Friday when shooting began. "I support the idea of an independent Donbas, but I have not really been out on the streets that much," he said. "I saw people shouting at the soldiers and went to have a look. One of them came out from behind a tree and opened fire. I was hit and I fell. Others then dragged me away.
"You have people in Kiev who claim to be a government, but then send tanks [armoured personnel carriers] to attack the citizens of their country. Would you want to be part of such a country? People are voting and they'll vote to leave Ukraine."
Just how easily people would be able to vote in today's referendum on secession was open to doubt, as was the effect the result might have.
Roman Lyagin, the 33-year-old head of the "election commission" of the Donbas People's Republic was confident that "there will be around 20,000 willing to work as election officials, the voter turnout will be around 70 per cent and preliminary results will be announced a few hours after the polls close".
The reality is that the voting list is two years old and no minimum turnout will be required to turn the result into constitutional changes designed to end the state of Ukraine in its present form. There will be no outside observers and no challenges will be permitted.
However, for those who complain about the mounting costs of elections in the West, the People's Republic provides an extremely cheap alternative: just over €1,200 (£980) has been spent, €500 of which went on toners for three borrowed printers for ballot papers.
The question on the paper is: "Do you support the act of self-rule by the People's Republic of Donetsk?" Some residents of the city were puzzled by what that meant. "Is it autonomy or independence? I am against independence but want autonomy in a federal system; this has not been explained," said Eliana Korchinova, a clerical assistant.
Will she vote? "No one has told me where to go to vote. I am not sure they have actually built the polling stations yet."
Germany and France warned Russia yesterday that it would face a further round of sanctions if the Ukraine presidential elections did not go ahead on 25 May.
Chancellor Angela Merkel and French President François Hollande said that the country faced destabilisation if the vote did not occur.
They also called for a visible reduction in Russian forces on the Ukraine border, which Nato has estimated at some 40,000 troops, and warned referendums scheduled to be staged by pro-Russian separatists in eastern Ukraine today were illegal.
The European Union agreed a round of sanctions on 6 March but at a joint press conference Ms Merkel said, "We would be ready to take further sanctions against Russia if the [presidential] elections in Ukraine fail."
Germany, which relies on Russia for 40 per cent of its natural gas, has previously appeared reluctant to broaden sanctions but Ms Merkel now appears more concerned about instability in the region and beyond if the Ukrainian crisis continues.
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