Pakistan's President Pervez Musharraf has offered to help India investigate the train bombings in Mumbai, which killed some 200 people.
There have been sharp exchanges between India and Pakistan
Gen Musharraf condemned the loss of "precious lives" in the attacks.
India and Pakistan have exchanged barbed remarks over unproven suspicions that Kashmiri militants might have played a part in the attacks.
Indian Prime Minister Manmohan Singh is expected to visit Mumbai on Friday, where he will meet some of the injured.
On Thursday evening, Bollywood stars joined hundreds of Mumbai residents in a march of solidarity over the attacks.
Shabana Azmi, an actress and organiser of the march, urged authorities to do all they could to find those responsible.
Police hunting the bombers have released sketches of three suspects wanted in connection with Tuesday's attacks.
Gen Musharraf was among the first world leaders to offer his condolences to India in the wake of the bombings, in which seven commuter trains were bombed in less than 15 minutes.
Commuters have been paying their respects to the dead
But in the days since, there have been sharp exchanges between government ministers in India and Pakistan.
India reacted angrily to reports that Pakistan's foreign minister had drawn a link between the bombings and lacklustre progress in peace talks over the disputed region of Kashmir.
Later Pakistan's foreign minister dismissed Indian calls for Pakistan to rein in militants operating from its territory.
In an interview with a Pakistani TV station on Thursday night, Gen Musharraf said: "Whosoever has done this cannot be pardoned at all...
"I assure Prime Minister Manmohan Singh that the Pakistan government and I myself are with him in any investigation he wants to carry out," he said.
Meanwhile, in Mumbai, formerly known as Bombay, the investigation into the bombings continues.
Police carried out hundreds of raids in the city and other parts of the state of Maharashtra, and some 300 people were brought in for questioning.
But no formal arrests have been made in the city, and many of those detained have now been released.
The home minister of the state of Andhra Pradesh, K Jana Reddy, told reporters that a man on a train in Hyderabad - identified as Abdullah - was arrested in connection with the bombings.
A Muslim organisation banned in India, the Students' Islamic Movement (Simi), is the latest group to deny involvement in the attacks.
At a news conference in the capital Delhi, a Simi leader, Shahid Badr Falahi, described the attacks as deplorable and said his organisation had had no part in them
Activists of Simi were allegedly involved in bomb blasts in Mumbai in 2003, when 55 people were killed. They are currently being tried in a special court in the city.
The anti-terror squad chief, KP Raghuvanshi, and the top bureaucrat in Maharashtra state, DK Shankaran, have both said the attacks have similarities with previous attacks committed by Lashkar-e-Toiba, a leading Pakistan-based militant group fighting in Kashmir.
But the group has not been directly accused, and has strongly denied any involvement.
Actors join march
On Thursday evening, Bollywood stars such as Anil Kapoor and Hema Malini joined hundreds of Mumbai's residents in a symbolic march to a shrine of the father of non-violent protest, Mahatma Gandhi, reports the BBC's Madeleine Morris.
"Unless justice is ensured, unless the guilty are punished, the situation will get worse," said march organiser Ms Azmi.
Others urged the people of Mumbai not to respond with attacks against the city's Muslim community, and many said they wanted to show that Mumbai would not be cowed by violence.
As hundreds of people stood silently around Mahatma Gandhi's statue holding candles aloft, it certainly felt like a city more united than divided by terror, our correspondent says.