Britain has called on Pakistan President Pervez Musharraf to resign as head of the army and guarantee that "free and fair" democratic elections will take place next year.
Foreign Secretary David Miliband said it was "a defining moment" for the country and its relationship with the world.
He was speaking after fresh clashes and arrests in the wake of President Musharraf's declaration of a state of emergency on Saturday.
Mr Miliband said there was now a "unanimous view" in the international community that democracy and human rights needed to be restored.
Mr Miliband told reporters at the Foreign Office that he had spoken to his counterparts in France, Germany, Pakistan itself and US Secretary of State Condoleezza Rice.
"Now is the time for leadership from President Musharraf and from opposition leaders," he said. "Now is the time for President Musharraf to be absolutely clear that elections will go ahead on January 15 on a free and fair basis and that he will resign as head of the army."
At least 350 people were arrested as riot police clashed with lawyers in Pakistan on Monday. They joined another 500 opposition leaders and activists placed in custody as security forces across Pakistan blockaded courts to quash the protests which have been banned.
Britain and the EU said they were reviewing aid to Pakistan and the Dutch government froze its assistance programmes. But the Foreign Secretary added: "Now is not the time for threats to aid which is important to the Pakistani people."
Mr Miliband said: "President Musharraf has the opportunity to be absolutely clear about election dates, about his own position as a civilian leader. That sort of clarity can only help. The next few weeks are going to be critical in determining the international community's engagement with Pakistan across the whole range of issues."
And he warned: "I don't want to hide at all the gravity of the situation, or that this is a defining moment for Pakistan and its leadership. The whole world will be watching."
