Prime Minister David Cameron today condemned "despicable" scenes of violence in the Egyptian capital Cairo, where supporters of Hosni Mubarak have attacked demonstrators calling for the president's removal.
Speaking alongside United Nations Secretary-General Ban Ki-Moon after talks in 10 Downing Street, Mr Cameron said that the political transition to a new broad-based government in Egypt needed "to be accelerated and to happen quickly".
Central Cairo today witnessed the worst scenes of violence since anti-Mubarak demonstrations began more than a week ago, with several thousand supporters of the president, some riding horses and camels and wielding whips, attacking protesters in Tahrir Square.
Speaking outside Number 10, Mr Cameron said: "If it turns out that the regime in any way has been sponsoring or tolerating this violence, that would be completely and utterly unacceptable.
"These are despicable scenes we are seeing and they should not be repeated."
Mr Ban added: "I once again urge restraint to all sides... Any attack against peaceful demonstrators is unacceptable and I very strongly condemn it."
A military spokesman appeared on Egyptian state TV today asking protesters to disperse, in a sign that the Army's toleration of the demonstrations - which yesterday saw an estimated 250,000 people cram into Tahrir Square - may be coming to an end.
Mr Mubarak yesterday went on national television to say that he would step down at national elections in September, but rejecting opposition calls for him to go now.
Around 10,000 opponents of the regime returned to protest this morning, renewing demands for Mr Mubarak's immediate removal.
Chaotic scenes developed as they clashed with pro-Mubarak protesters, who tore down placards and posters and fought with demonstrators. The two sides hurled stones and lumps of concrete at one another and some of the pro-Mubarak forces were pulled from horses and camels and beaten. Some soldiers fired in the air in an attempt to control the crowd.
Mr Cameron said that he "completely" condemned the violence.
Today's events "underline the need for political reform and for that political reform to be accelerated and to happen quickly," he added.
"We need to see a clear road-map for that political reform, so people in Egypt can have confidence that their aspirations for a more democratic future with greater rights are met.
"That change needs to start happening now and the violence needs to stop."
Mr Ban said he was "deeply concerned" at the "unacceptable" violence.
The UN had been warning for a decade of the need for change in the Arab world, he said, and he offered the UN's assistance in smoothing the way for peaceful reform.
"It is important at this juncture that it is ensured that an orderly and peaceful transition should take place," said Mr Ban.
"I urge all parties to engage in such a dialogue and such process without any further delay.
"We should not under-estimate the danger of instability across the Middle East.
"The United Nations stands ready to provide any assistance to such reform efforts by Egypt and any countries in the Arab (world)."
He urged Egypt's leaders to "accept the will of their people that they should listen more attentively to the genuine and sincere wish of the people".
And he said: "It is important that there needs to be a peaceful and orderly transition. If a transition is to be taken, this should be done now."