Washington and the U.N. Security Council condemned the violence, which Serbia blamed on "isolated vandals".
European Union foreign policy chief Javier Solana said on a visit to Slovenia that it was "the obligation of the country" to protect foreign embassies.
"Things will have to calm down before we can recuperate the climate that would allow for any contact to move on the SAA (Stabilisation and Association Agreement)," he said of a preliminary deal on ties with the European Union.
The pact was initialised last year but the European Union has said it will not sign it until Belgrade fully cooperates with the U.N. war crimes tribunal for the former Yugoslavia.
Russia, Serbia's ally and critic of Kosovo's secession, said Western states should have anticipated the backlash.
"People who advocated a unilateral proclamation of independence for Kosovo, should have calculated the consequences of this step," Russian Foreign Ministry chief spokesman Mikhail Kamynin was quoted as saying by RIA news agency.
Rallies were planned for Friday in the southern city of Nis, the northern Kosovo town of Mitrovica and the Montenegrin capital Podgorica, organised by the neighbouring country's 30-percent ethnic Serb minority.
Serbs in former Yugoslavia have been protesting against Kosovo's independence, which was declared on Sunday and quickly recognised by Washington and most EU states.
On Thursday, about 200,000 people attended a peaceful state-backed rally, listening to speeches and songs about what they consider Serbia's historic heartland in a mostly melancholy atmosphere.
But police were nowhere to be seen when a few hundred rioters attacked the U.S. embassy for the second time in a week.
A crowd of about 1,000 cheered "Serbia, Serbia" one ripped the Stars and Stripes off its pole and others jumped up and down on a balcony, holding a Serbian flag.
Smoke billowed out of the embassy as papers and chairs were thrown out of windows and burning doors were wedged in the frames.
American officials said only security personnel were at the embassy at the time, in a different area. Riot police arrived later and fired teargas to disperse the crowds.
Hospital officials said around 150 people were injured in street clashes, including 30 police and some journalists.
European and U.S. leaders criticised Serbia for failing to dispatch police quickly. "I only wish the Serbian police had intervened more quickly," said German Defence Minister Franz Josef Jung in Slovenia.
Serbia waged a diplomatic campaign against Kosovo's secession, condemning its swift recognition by the U.S. and major EU countries such as Germany, France and Britain.
"As long as we live, Kosovo is Serbia," Prime Minister Vojislav Kostunica told the rally.