The UN Human Rights Council has strongly condemned the violence in Syria and is to appoint a special investigator on the crackdown on anti-government protesters.
A council report on the violence is to be sent to the UN Secretary General.
The UN estimates 4,000 people have been killed during a crackdown on anti-government protests.
The UK ambassador said it was the toughest resolution ever passed by the Geneva-based council.
Earlier, UN human rights chief Navi Pillay called for "urgent" action to protect civilians in Syria.
But Syria's ambassador in Geneva told the council a solution to his country's problems could not be found by the international community.
Any UN intervention would simply deepen the crisis, he warned.
'Crimes against humanity'
The resolution was approved at an emergency meeting of the council by 37 votes in favour to four against, with six abstentions.
Those voting against included Russia and China, who have resisted moves for a UN Security Council resolution on Syria, for fear it could lead to a military intervention such as the one in Libya.
"The positions [adopted] in the document, which include the veiled hint of the possibility of foreign military intervention under the pretext of defending the Syrian people, are unacceptable to the Russian side," Russia's foreign ministry said in a statement, quoted by the AFP news agency.
The resolution demands the suspension of security forces suspected of violations and the release of prisoners of conscience.
However, some human rights groups are disappointed the council did not make a clearer call for referring Syria to the International Criminal Court (ICC), as Ms Pillay had wanted.
A report for the UN earlier this week said security forces had committed crimes against humanity in Syria.
"The Syrian authorities' continual ruthless repression, if not stopped now, can drive the country into a full-fledged civil war," Ms Pillay warned.
"In light of the manifest failure of the Syrian authorities to protect their citizens, the international community needs to take urgent and effective measures to protect the Syrian people," she told the council.
Ms Pillay said at least 4,000 people had been killed, including 307 children, and tens of thousands arrested in the unrest.
Correspondents say there is unusual unity among council member states, with Arab nations, Europe and the US all backing pressure on Mr Assad.
The US ambassador Eileen Chamberlain Donahoe told Reuters news agency the resolution "set the stage" for further action by the UN and other institutions.
Asked if this included the ICC, she replied: "Absolutely, including the ICC if the Security Council chooses to refer this matter."
Meanwhile, there has been no let-up in anti-government demonstrations in Syria, the BBC's Jim Muir reports from neighbouring Lebanon.
Protesters - who regularly take to the streets after Friday prayers - have dubbed the latest protest "the Friday of the no-fly zone", a reference to the safe haven they hope the outside world will impose. Activists say at least six people have died.
Heavy shooting was reported near the border with Lebanon in the town of Talkalakh. Activists said at least two people were wounded on the Lebanese side of the border, including an 11-year-old girl.
Seven soldiers were also killed, after army defectors attacked an intelligence base in Idlib, a spokesman for the group told the BBC.
In Damascus, meanwhile, thousands of loyalists held rallies in support of Mr Assad, our correspondent reports.
Impact on oil
Earlier, French Interior Minister Claude Gueant said the authorities were tightening security for Syrian opposition leaders based there, because of threats.
Members of the opposition Syrian National Council, including its leader, Burhan Ghalioun, are based in Paris.
On Thursday, the European Union tightened sanctions against Mr Assad's government, placing bans on exporting gas and oil industry equipment to Syria and trading in Syrian government bonds.
It also expanded a list of companies and individuals which face assets freezes and travel bans.
Royal Dutch Shell said it would stop its oil operations in Syria to comply with the EU sanctions. Shell is a minority partner in Syria's state-owned Al Furat Petroleum Company, which has been added to the sanctions list.
However, French oil company Total is to continue oil production in Syria for now, as its Syrian joint venture partner is not on the sanctions list.