Recognition of Kosovo's independence has gained momentum with the US, France, the UK, Germany and Italy all pledging their support.
EU states meeting in Brussels agreed that Kosovo should not set a precedent for other states, with Spain and others concerned about separatism.
Serbia has recalled its ambassador to the US in protest at recognition and threatened to withdraw other envoys.
Its president is to ask the UN to condemn the independence declaration.
Boris Tadic will ask the Security Council to annul the independence declaration and Belgrade is counting on Russia to veto Kosovo joining the UN as a new nation.
In Belgrade, about 10,000 students marched in protest, and Serb enclaves inside Kosovo also saw anti-independence rallies.
Serbian security forces were driven out of Kosovo in 1999 after a Nato bombing campaign aimed at halting the violent repression of ethnic Albanian separatists.
The province has been under UN administration and Nato protection since then.
Pledges of support
Monday saw Washington formally recognise Kosovo as a "sovereign and independent state".
STANCE ON RECOGNITION
For: Germany, Italy, France, UK, Austria, US, Turkey, Albania, Afghanistan
Against: Russia, Spain, Romania, Slovakia, Cyprus
In Brussels, pledges of recognition came after Monday's meeting of foreign ministers of EU states.
UK Foreign Secretary David Miliband said a desire to establish a unified position after "the disunity of the 1990s" had dominated the meeting in Brussels.
As for the UK's formal recognition, he added:
"The plan is to do it this evening and diplomatic relations will then be established... and in the course of the next days and weeks all the items of full diplomatic representation will be put into place."
Population about two million
Majority ethnic Albanian; 10% Serb
Under UN control since Nato drove out Serb forces in 1999
2,000-strong EU staff to take over from UN after independence
Nato to stay to provide security
After a lot of soul-searching, the EU forged a semblance of unity, BBC European Affairs correspondent Oana Lungescu writes.
It adopted a compromise proposal from Spain, one of several countries which argue that Kosovo's independence is a breach of international law and will boost separatists everywhere.
To ease those concerns, the EU's statement says that Kosovo does not set a precedent and leaves it up to each member state to decide on their future relations.
Most EU countries led by the biggest among them will take this as a green light for recognition, our correspondent notes.
Some of the EU states which did not recognise independence have experienced separatist unrest of their own, such as Spain with its Basque region and Cyprus with its Greek-Turkish division.
The EU earlier agreed to send about 2,000 police, justice and civil administration officials to oversee Kosovo and help develop the province's institutions.
Speaking to Serbian TV from New York, the Serbian president, Mr Tadic said he intended to "demand from [UN Secretary General] Ban Ki-moon the immediate annulment of the independence proclamation by the non-existent state in Kosovo".
HAVE YOUR SAY
This is an ugly victory for demographic warfare
The recall of the Serbian ambassador to Washington was announced to parliament in Belgrade by Prime Minister Vojislav Kostunica.
"America violated international law for its own interests," he said.
It was, he added, the "first urgent measure of the government which will be implemented in all countries that recognise unilateral independence".
Serbia's interior ministry filed criminal charges on Monday against Kosovo Albanian leaders instrumental in proclaiming independence.
It accused Prime Minister Hashim Thaci and two others of proclaiming a "false state" on Serbian territory.
Chanting and playing music, students marched in Belgrade under a huge banner reading "Kosovo is a part of Serbia and Serbia is a part of the world".
The scenes were in stark contrast to riots that took place in the capital a few hours earlier when a few hundred people, mainly football hooligans, went on the rampage clashing with police and stoning embassies, the BBC's Nick Hawton reports.