Although the cause of the blast was unknown, there were inevitable suspicions that it was a bomb, as it came only four days after a candidate for the city's deputy mayor's post, Valery Shantsev, an ally of President Boris Yeltsin, was critically injured when a packet of TNT blew up outside his apartment.
Reports said the explosion happened as the train approached Tulskaya station in the southern sector of the metro system, in which security has been beefed up for 18 months because of the war in Chechnya.
The incident may mean stricter controls will have to apply for Mr Yeltsin's appearance in Red Square today for one of the final flourishes to his campaign, a rally to whip up enthusiasm for Sunday's election among Russia's young people.
The rally coincides with a holiday marking Russia's declaration of independence six years ago, an occasion which Mr Yeltsin clearly aims to use as a platform to drum up further support in his effort to defeat his Communist rival, Gennady Zyuganov.
If he still appears, he will exploit it to the full. The organisers expect the president to appear on stage alongside several popular Russian rock bands. The audience will be bombarded with messages telling them to "choose or lose". Pictures will be broadcast nationwide on state television. The Independence Day gala is a reminder of how keen the Yeltsin campaign is to mobilise the young vote, which tends to be anti-Communist.
"A large number of young people stayed home during December's parliamentary elections," said Dimitry Tugarin, spokesman for the organisers. "We have to get them out this time."
Nor is this the Yeltsin team's only anxiety. There are signs that some presidential handlers think his campaign is overheating, not least because of his lead in the polls. Mr Yeltsin's boast that he will win the first round outright by getting more than 50 per cent has led to worries that voters will conclude he is assured of victory and opt for a third candidate - or not vote.
In what may have been a move to dampen down over- optimism, Yakov Borovoy, press director at his campaign headquarters, yesterday produced figures predicting Mr Yeltsin would lose to Mr Zyuganov in the first round - with 26 per cent to the Communists' 28 per cent.
This contradicts claims by Sergei Filatov, one of the top coordinators of the President's team, who expected to see Mr Yeltsin with 30-35 per cent support by the end of this week. Mr Borovoy said the number of undecided voters had narrowed to a core of about 15 to 20 per cent of the electorate, dominated by poorly educated, low-income, elderly women.
If Mr Yeltsin is beaten by Mr Zyuganov, it will alarm supporters in Russia and the West, but it should be no surprise. The Communist-nationalist coalition has a strong grassroots organisation throughout most of provincial Russia.
There are other election day headaches. The first round coincides with the Euro 96 football match between Russia and Germany, which could easily lure voters away.
But if Mr Yeltsin does come second, it will not be for want of trying to win. He was in the southern city of Novocherkassk yesterday, shrewdly reminding voters of the horrors of Communism by promising to build a monument to 23 people who were shot by the Soviets for taking part in a demonstration over food shortages in 1962.
Mr Zyuganov has been criticised for being dull, but these days it is truer to describe him as plain odd. Last Sunday he held a rally, featuring peels of church bells, a parade of icons, and scantily clad majorettes, at which he compared Mr Yeltsin to Satan.
"Let's remember what is in the Apocalypse," he said . "The Devil has sent two beasts from hell. The first has a mark on his head (a reference to Mikhail Gorbachev), and the second has a mark on his hand" - a reference to Mr Yeltsin, who is missing two fingers.
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