Gordon Brown tonight vowed a "vigorous response" to attacks in Mumbai which have left 78 people dead.
The Prime Minister promised to back the Indian government as it responds to the series of incidents carried out by heavily-armed gunmen.
"These outrageous attacks in Mumbai will be met with a vigorous response," Mr Brown said in a statement.
"I have sent a message to (Indian) Prime Minister (Manmohan) Singh that the UK stands solidly with his government as they respond, and to offer all necessary help.
"Urgent action is under way to offer every possible protection to British citizens in the region."
In the US, a State Department spokesman said: "We strongly condemn the terrorist attacks that have taken place in Mumbai, India.
"Our sympathies go out to the families and friends of those killed and injured, and to the people of Mumbai.
"We are monitoring the situation very closely and stand ready to support the Indian authorities as they deal with this horrific series of attacks.
"At this point, we are unaware of any American casualties."
Foreign Secretary David Miliband also spoke out against the attacks.
"Today's attacks in Mumbai which have claimed many innocent victims, remind us, yet again, of the threat we face from violent extremists," he said in a statement.
"I condemn these attacks unreservedly. Our thoughts are with the families and friends of those killed and injured.
"The UK and India will continue their joint efforts to counter the actions of terrorists."
Middlesex skipper Shaun Udal said that that team has cancelled plans to fly out to Mumbai tomorrow for the inaugural Twenty20 Champions League in the wake of the attacks.
Udal confirmed they were postponing their plans by at least 24 hours, telling Sky Sports News: "All we know is our flight has been cancelled in the morning."
Udal said he understood the tournament was still set to go ahead but with the Mumbai matches moved to Bangalore.