Lebanese opposition party Hezbollah has condemned a UN Security Council vote to set up a tribunal to try suspects in the killing of Rafik Hariri.
Hariri's killing sparked protests that pressured Syria to leave Lebanon
The ex-PM and 22 other people died in a huge car bombing in Beirut in 2005.
A UN inquiry has suggested that Syrian and Lebanese intelligence forces played a role - which Syria denies.
The pro-Syrian Hezbollah group, which has previously blocked a parliamentary vote on the plan, said the UN decision violated Lebanese sovereignty.
"The resolution, as much as it constitutes a violation of the Lebanese state and an attack on its internal affairs, it is contrary to legal rules and the charters and principles of the United Nations and the objectives for which it was established to achieve," the statement said.
The resolution was "illegal and illegitimate at the national and international level", the group added.
Lebanon's pro-Syrian president, Emile Lahoud, has said he fears the tribunal will cause fresh division in the country.
Syria has also condemned the decision.
Hezbollah's opposition to the tribunal has so far meant that Lebanese PM Fouad Siniora has been unable to get a vote backing the plan through parliament.
The UN resolution gives the Lebanese parliament one more opportunity to back the plan.
If the parliament does not ratify the tribunal by 10 June, the Security Council may independently authorise a tribunal, as it did in Rwanda and the former Yugoslavia.
The issue of the tribunal has been at the centre of a political crisis in Lebanon which has seen the opposition withdraw its ministers from the cabinet, and attempt to bring down the government through street protests.
The Syrian ambassador at the UN, Bashar Jaafari, condemned the UN vote.
"Definitely this is something that goes against the interests of the Lebanese people and Lebanon as a whole," he told reporters after the vote.
The Syrian government has in the past threatened not to co-operate with an international tribunal.
NEXT STEPS FOR TRIBUNAL
Lebanon must ratify tribunal by 10 June, or UN could push ahead regardless
May take about a year to establish court
Tribunal to be held in undetermined neutral location
Syria has threatened not to co-operate with the tribunal
The resolution was adopted by 10 votes to zero, with five abstentions from Russia, China, South Africa, Indonesia and Qatar.
Syria has always strongly denied any involvement in Mr Hariri's death, but in 2005 it withdrew its troops from Lebanon after a presence of 29 years, following massive domestic and international pressure after the assassination of Mr Hariri.
Four pro-Syrian Lebanese generals have been under arrest for more than a year, accused of involvement.
The Syrian government position is that the tribunal will be a violation of Lebanese sovereignty and that it will further undermine stability there.
As far as its alleged involvement in the assassination is concerned, Syria says that if one of its citizens was to be formally accused of involvement, that person could be tried only by Syrian courts.
Saad Hariri, the son of the assassinated politician, said the tribunal was a turning point for the country and "an opportunity for all Lebanese to unite".
Speaking live on Lebanese television, he said the time had come for justice.
Some of the abstaining ambassadors at the Security Council objected to the reference in the resolution to Chapter Seven of the UN Charter, used in cases of threats to international peace and security.
However, the US and its allies argued that only Chapter Seven resolutions were legally enforceable. There is a hope in Lebanon that the resolution will end a long, sad history of unpunished political assassinations, says the BBC's Kim Ghattas in Beirut.
The issue of the tribunal has caused deep divisions in Lebanon
Reporting from the Damascus, Magdi Abdel Hadi says the new resolution is likely to increase the tension between Syria and the Western-backed Lebanese government of Mr Siniora. It is also likely to increase polarisation inside Lebanon itself.
People in Lebanon are on edge after a series of bomb attacks last week and an ongoing bloody standoff in northern Lebanon between the army and militants from the Fatah al-Islam group.
A Lebanese army spokesman said a soldier was killed on Thursday by sniper fire from Fatah al-Islam militants inside the Nahr al-Bared Palestinian refugee camp.
At least 80 people have been killed in the fighting, including 35 soldiers, 20 civilians and at least 25 militants.