Underlining global solidarity with India, Brown stressed that the international community has to ensure that there are 'no safe havens for terrorists and no hiding places for those who finance terror attacks'.
"We know the group responsible is LeT (Lashkar-e-Toiba), and they (Pakistan) have a great deal to answer for," Brown told reporters when asked whether Pakistan was doing enough to crack down on anti-terror outfits working against India.
"We know that some arrests have been made and some people have been held even as we speak," said Brown after holding talks with Indian Prime Minister Manmohan Singh here.
"I hope to convey India's concerns when I meet President (Asif Ali) Zardari," Brown said as he wrapped up his brief visit to India.
"I am also visiting Islamabad. I will explain the concerns Indian people have," said Brown, minutes before he left for the Pakistani capital.
The LeT, which was banned in 2002 and declared a terrorist organization, is widely seen as a creation of Pakistan's spy agency Inter Services Intelligence (ISI) to wage a proxy war to challenge Indian rule in Jammu Kashmir.
"The important thing to recognise is that President Zardari has made it clear that he wishes to do all he can to counter terrorism and work together with India," the British Prime Minister said, adding that he will be able to tell more after talks with Zardari and his top aides later in the day.
Brown, held talks with Manmohan Singh and his advisers over breakfast on Sunday, Dec 13 morning.
Brown, the first foreign head of state to visit India in connection with the Mumbai attacks, assured India of the support of the international community in dealing with the outrageous terror that is suspected of having trans-national links.
"I have told the prime minister that India has support of the whole world. We will work together in tackling terrorism. We will work together to build international support in tackling terrorism," Brown stressed.
'We will work together on issues of security. You will get support of many members of the international community. I wanted to come to India and give my condolences first hand at this terrible outrage in Mumbai that has shocked the world," he said.