By Ian Dunt
David Cameron has praised demonstrators and warned the military to make way for civilian leadership on a historic day for Egypt.
The comments come as President Hosni Mubarak finally stepped down following three weeks of protests.
There were remarkable, historic scenes in Cairo and across the country this evening, as a wave of euphoria swept the streets.
The Supreme Military Council of Egypt is now in charge of the country until elections are held.
Mr Cameron struck an optimistic but cautious note when he commented on the developments, urging the interim military government to make way for civilian leadership.
"Today has been a remarkable day, particularly for those people in Egypt who spoke out so bravely for their country," the British prime minister said.
"As a friend of Egypt and the Egyptian people, we stand ready to help in any way we can.
"It must be a government that starts to put in place the building blocks of a truly free, open and democratic society.
"Those who now run Egypt have a duty to reflect the wishes of the Egyptian people."
Elsewhere, there was a gushing reaction from British figures across the political spectrum.
Shadow foreign secretary Douglas Alexander commented: "Eighteen days of protests have ended a thirty year presidency.
"The jubilation of people in Tahrir Square will echo around the world and marks the extraordinary achievement of an aggrieved people.
"But now we need clarity and responsibility from the military, with rapid progress towards the democratic future that Egyptians have so clearly demanded."
EU foreign secretary Baroness Ashton told the BBC: "People are giving a very clear message. They want to see democracy. They want to see human rights. It's our responsibly to back that."
Green MP Caroline Lucas tweeted: "Great news that Mubarak has gone.
"Hope rest of regime swiftly join him, protestors are immediately released & real change follows."
Human rights activist Peter Tatchell was more cautious, tweeting: "Mubarak has gone via a military coup. Police state remains."
Amnesty International UK commented: "One man leaves but the corruption, torture & inequality remain. Human rights must be at heart of Egypt's future."
In Tunisia, where demonstrations triggered a wave of revolution which today brought down a 30-year-old dictatorship, crowds waved Egyptian flags in solidarity.
It became increasingly clear that events in Egypt had reached boiling point today, despite a tough speech from the former president last night, in which he swore to stay in power until September.
Even a statement in support of the status quo from the military failed to stop the momentum of the movement.
Deputy prime minister Nick Clegg admitted the "genie is out the bottle" in an interview with the Daily Telegraph, as leaders across the West were reduced to a bystander role while crowds of protestors took to the streets.
"I think many people will share the frustration of many of the people who are demonstrating in Cairo right now," Mr Clegg said.
"Change is coming in Egypt, the question now is, how quickly, how peacefully and how democratically."
Crowds burst forth from Tahrir Square this afternoon, after huge numbers of ordinary Egyptians took to the streets. Crowds spread to the presidential palace and state TV headquarters.
Western journalists on the ground reported that nearly every street in the Egyptian capital was swarming with demonstrators.
Former foreign secretary David Miliband struck a more cautious note in the build up to the resignation.
"Very dangerous moments now in Egypt," he tweeted.
"Army rule rarely ends well - because they always find reasons not to end it."
President Barack Obama immediately convened a security meeting following President Mubarak's address last night, after which he delivered the strongest rhetoric yet from Washington.
It was "imperative that the government not respond to the aspirations of their people with repression or brutality", the US president said.
But in reality, the Egyptian crisis showed how quickly the US' strength was in decline. Despite pressure from Washington, the former president did not step down. On the other side of the conflict, America was caught completely unprepared by the protests and struggled to form a coherent or principled view during the revolution.
The former president is thought to be at his palace in Sharm el-Sheikh after helicopters were seen leaving the presidential palace in Cairo.