David Cameron and US President Barack Obama have called for an "orderly transition" to a democratic government in Egypt.
As the crisis engulfing President Hosni Mubarak escalated, the two leaders held talks and gave the strongest indication yet that they do not envisage him hanging on to power.
The military staged a show of strength on the sixth day of mass demonstrations demanding an end to Mr Mubarak's three-decade rule. But protesters gave no sign of dispersing despite more than 100 deaths so far, with huge crowds again roaming the streets of major cities.
Foreign Secretary William Hague stressed that Egypt was a "sovereign nation" and Britain was not trying to "pick who should be president". However, he insisted there should be smooth and peaceful reform, raising concerns that extremists could seek to exploit any power vacuum.
Mr Hague indicated that the UK would be concerned if 82-year-old Mr Mubarak was replaced by the Muslim Brotherhood, a conservative religious group seen as one of the main opposition forces.
Gangs of armed men are reported to have stormed four jails across Egypt on Sunday, helping to free hundreds of Muslim militants and thousands of other inmates.
The army has sent hundreds more troops and armoured vehicles on to the streets of Cairo and other cities, and jets have been flying low over the main square in the capital. However, they appeared unwilling to take action against gangs of young men with guns and large sticks who were smashing cars and robbing people.
Mr Cameron personally spoke to the embattled president on Saturday to express his "grave concern" about violence against the anti-government dissidents. In a separate joint statement with French President Nicolas Sarkozy and German Chancellor Angela Merkel, the Prime Minister said: "The Egyptian people have legitimate grievances and a longing for a just and better future."
A Downing Street spokeswoman said Mr Cameron and Mr Obama had discussed their "shared and deep concern" at the crisis in a telephone call on Sunday night. Mr Cameron also discussed the situation with King Abdullah of Jordan, a key powerbroker in the region. "The Prime Minister and President Obama were united in their view that Egypt now needed a comprehensive process of political reform, with an orderly, Egyptian-led transition leading to a government that responded to the grievances of the Egyptian people and to their aspirations for a democratic future," the spokesman said.
Nobel Peace Prize winner Mohamed ElBaradei, a leading opposition moderate and potential successor, joined the crowds in Cairo's main Liberation Square on Sunday night. He insisted the president had to go immediately.