UK authorities played down reports that the terrorists included Britons as violence in the city continued for a third day.
Gordon Brown said there was no mention of any of the terrorists being linked with Britain during a conversation with Indian Prime Minister Manmohan Singh.
He said: "At no point has the Prime Minister of India suggested to me that there is evidence at this stage of any terrorist of British origins but obviously these are huge investigations that are being done and I think it will be premature to draw any conclusions at all.
"We remain steadfast and firm standing with India and all other countries against any form of terrorist activity and we will be vigilant in both helping the Indian authorities and in making sure that in every part of the world we support those who are fighting terrorism."
But Indian Chief Minister Vilasrao Deshmukh claims two British-born Pakistanis were among eight gunmen arrested by Indian authorities, according to Associated Press reports.
Indian authorities said Mumbai would soon be back under their full control but confirmed the number of foreigners known to be killed in the attacks had risen to eight.
At least 143 people - including a wealthy British businessman - were left dead in the wave of terror that swept India's financial capital.
Earlier a German MEP caught up in the attacks said she had heard that British nationals were among the terrorists, as reports of the death toll hit 143, including eight foreign nationals.
Erika Mann was part of a trade delegation of MEPs from Brussels staying at the Taj Hotel.
Before leaving the city on a flight to London, she said she had escaped through an underground passage in the hotel.
She added: "The attacks appear to have a European dimension. We have heard from journalists and other people we were with that English citizens took part in the attacks and were killed in the hotel.
"This information is not confirmed, however. We were told that they came in by boat."
Home Secretary Jacqui Smith said UK authorities had "no knowledge" of any British links with the massacre.
She told reporters: "We will do anything we can to help Indian authorities through what is obviously a very difficult time."
Ms Smith added: "We will do what is necessary. At the moment the priority is to support the immediate needs.
"We will work with the Indians to see what we can learn from the events."
Ms Mann, a member of the Socialist group in the European Parliament, said hotel staff helped steer her towards a route away from the hotel as gunmen burst in and began firing.
"The terrorists started shooting behind us. When we got to the street I stopped a car and by luck it belonged to some Indian journalists. The Indian people were wonderful in their support."
She said a new approach was now needed to tackle global terrorism: "These attacks have taught us all a difficult lesson," she said.
"We cannot continue just with local and regional structures to fight terrorism when we face an enemy that is organised on a global scale. Global terrorism of the sort we experienced in Mumbai involves a wide range of people, from young people influenced by fundamentalism to business people.
"This cannot be left as a problem for India alone. The ordinary people are as fed up as anyone else."
Meanwhile, a top Indian official says eight foreigners are known to have been killed so far in the Mumbai attacks and 22 more have been injured.
Top security official M.L. Kumawat says the dead include three Germans, and one person each from Japan, Canada and Australia. The nationalities of two more are unknown.
The injured include five from Britain, three Germans, two Americans, two from Oman and one each from Norway, Spain, Canada, Finland, Philippines, Australia, Italy and China. Two more were unknown.
24 more bodies were found today, taking the death toll to 143.
Even after claims by Indian authorities that the situation was under control, explosions were still being heard in central Mumbai.
Prime Minister Gordon Brown said that as far as the Government was aware, there were no British hostages still being held in the Indian financial capital.
And he said ministers were determined to do what they could to help the Indian authorities dealing with the attacks.
"It is clear that we have got to help the Indian government deal with this terrorist incident and we have sent people from the Metropolitan Police to help," he said.
"This is an atrocious outbreak of violence planned in advance and I think we have got to look at how international action against terrorism can be improved over the next period of time."
Mr Brown said the British High Commissioner to India, Sir Richard Stagg, was trying to establish how many UK nationals had been injured in the attacks.
He said his thoughts were with those who had died, including the British businessman Andreas Liveras.
"Of course, we are mourning the death of a British citizen who died and there are a number of British citizens who are injured," he said.
"As the High Commissioner visits different hospitals in the Mumbai area, he is trying to discover the degree to which people are ill and injured."
Mr Liveras, the founder of a luxury yacht business who was in his 70s, was pronounced dead on arrival at St George's Hospital in the city.
External affairs minister Pranab Mukherjee said "elements in Pakistan" were behind the Mumbai terror attacks.
He said: "According to preliminary information, some elements in Pakistan are responsible for Mumbai terror attacks.
"Proof cannot be disclosed at this time."
Hundreds were injured when the suspected Islamic militants attacked 10 sites in Mumbai.
Briton Mark Abell, whose home town was not known, was rescued at the Oberoi today.
He said: "I'm going home, I'm going to see my wife."
Mr Abell said he locked himself in his room during the siege.
He added: "These people here have been fantastic, the Indian authorities, the hotel staff. I think they are a great advertisement for their country."
Another man, who said he was British but would not give his name, said: "I didn't see anything. I just heard loud blasts.
"I was in my room. I didn't get out until an hour ago."
Gunmen launched co-ordinated attacks on targets in central Mumbai, including the two hotels and the city's packed railway station, on Wednesday.
Heavy gunfire continued today as commandos were dropped by helicopter on to the roof of a Jewish centre where at least 10 hostages were being held.
The commando attack on the centre run by the ultra-orthodox Jewish outreach group Chabad Lubavitch was punctuated by gunshots and explosions from within the building as forces cleared it floor by floor.
Security officials insisted their operations were almost over.
But within an hour of the statement two loud explosions were heard at the luxury Taj Mahal hotel, which authorities insisted last night was cleared of gunmen.
British citizen Nicole Griffen said she was rescued by Indian special forces from the Taj Mahal.
She told the BBC's Radio 5 Live: "They entered and looked through our passports and scouted around to see if there were (anyone) harbouring terrorists or attackers and then we were promptly told where to go by the central stairway and again we were asked to wait with other guests while they checked other floors and checked other rooms and we all congregated into one space where they could protect us centrally."
More than seven other Britons were among the injured, it was believed.
Retired teachers Diane and Michael Murphy, who live near Hexham, Northumberland, were on holiday in India and had only been in Mumbai for one day when the terrorists struck.
A group calling itself Deccan Mujahideen has claimed responsibility for the attacks.
But experts said they had never heard of the organisation and suggested that the radical Students Islamic Movement of India (Simi) might have been behind the massacre.
Prime Minister Singh blamed "external forces" for the violence - a phrase sometimes used to refer to Pakistani militants, whom Indian authorities often blame for attacks.
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