He was unveiling a £6m ($8.9m) deal with Pakistan he called "the most comprehensive anti-terrorist programme" between the UK and another country.
He is in Pakistan meeting President Asif Ali Zardari, following talks with India's premier Manmohan Singh.
UK police want to quiz a Pakistani suspect in the Mumbai attacks.
They want to interview Mohammed Ajmal Amir Qasab - the sole gunman taken alive - about terror groups operating from Pakistan.
Mr Brown's series of meetings in the region come as Pakistan accused India of violating its airspace.
India has denied this but has announced a security overhaul. It blames Pakistan-based militants for the Mumbai attacks, which killed at least 170 people, including one Briton.
Mr Brown said he had wanted to express his condolence for the Mumbai attacks at first hand.
'Work together'
He said the "terrible terror outrages" had shocked the whole world and the Pakistan-based Lashkar-e-Taiba (LeT) militant group, thought to be responsible, had "a great deal to answer for".
He added: "No country should have to go through what India has had to go through as a result of the Mumbai outrages.
Mohammed Ajmal Amir Qasab
British police want to interview Mohammed Ajmal Amir Qasab
"I've said to Prime Minister Singh we will give every help that we can. We will work together in tackling terrorism. And we will work together on issue of security. We also know that there have been arrests in Pakistan.
"We also know that the group responsible is LeT and that they have a great deal to answer for."
India has urged Pakistan to take action over the recent attacks.
Pakistan denies any involvement, but has promised to co-operate with the Indian investigation.
Mr Zardari has pledged to take "strong action" against terrorists, but has also called on India to share more information about the attacks.
Partnership plan
During a news conference in Islamabad with Mr Zardari, Mr Brown proposed the start of a new partnership with Pakistan to fight terrorism.
The "pact against terror" funding will go towards anti-car bomb equipment and material to educate people out of becoming extremists, he said.
"The time has come for action and not words, and I want to help Pakistan and other countries root out terrorism.
Gordon Brown meets his Indian counterpart Manmohan Singh in Delhi
"In return for this action we will continue to expand our counter-terrorist assistance programme with Pakistan, and it will be more than ever, the most comprehensive anti-terrorist programme Britain has signed with any country."
Speaking during a surprise visit to Afghanistan on Saturday, Mr Brown said wherever there was terrorism, it had to be fought.
He described Pakistan's border region with the country, where he met troops fighting the Taleban, as one end of a "chain of terror" that could stretch to Britain if more was not done to tackle the threat of al-Qaeda.
The prime minister's visit to Afghanistan came a day after four Royal Marines were killed in two separate bomb attacks.
Mr Brown spoke of his "disgust and horror" at the willingness of the Taleban to use a 13-year-old child to deliver a bomb in a wheelbarrow to a Marine patrol, killing three men and the boy.
The PM is expected to update MPs on the security situation in Afghanistan in an oral statement to the House of Commons on Monday.
Track with co.mments
