UN investigators said they had also found evidence of Lebanese collusion in Mr Hariri's death last February.
The report, prepared at the request of UN Secretary General Kofi Annan, says the car bombing was carried out by a group with extensive organisation.
Many in Lebanon blamed Syria for the killing, a claim denied by Damascus.
Troops have been deployed in the Lebanese capital, Beirut, and other cities in case of violence in the wake of the inquiry's findings.
'Lebanese collusion'
The UN team's report said there was "converging evidence pointing at both Lebanese and Syrian involvement in this terrorist act".
Mr Hariri's assassination - for which the likely motive was political - was complex and had obviously been planned for months, the report says.
As such, it "could not have been taken without the approval of top-ranked Syrian security official and could not have been further organised without the collusion of their counterparts in the Lebanese security services".
The inquiry established that "many leads point directly towards Syrian security officials as being involved with the assassination".
The investigators, led by German magistrate Detlev Mehlis, say the Syrian authorities only co-operated to a degree and accuse several interviewees of "trying to mislead the investigation".
They also accuse Syrian Foreign Minister Faruq al-Shara of lying in a letter sent to the commission.
The investigation is not complete, the report said, with several lines of inquiry still to be pursued.
'Impending crisis'
The UN Security Council will be briefed on 25 October by Mr Mehlis, who handed over the report to Mr Annan on Thursday.
John Bolton, the US ambassador to the UN, said several countries had been consulting on what steps to take based on the report's findings.
"We're going to study it very carefully, and based on what's in there, decide what our next course of action might be," he said.
The BBC's Ian Pannell in Damascus says it is widely believed that the Syrian authorities were angered by Mr Hariri's growing opposition to their influence in Lebanon.
Although Syria's President Bashar Assad constantly denies any involvement in the killing, the report will add to the sense of impending crisis felt in the country, our correspondent says.
Syria finds itself almost completely isolated, with little support from other Arab nations, and faces the prospect of crippling UN sanctions, he adds.
Syria was the main power in Lebanon until its military withdrawal earlier this year.
Since Mr Hariri was killed on 14 February a series of bomb attacks have targeted anti-Syrian journalists and politicians as well as Christian areas in Lebanon.
Mr Hariri's son, Saad, has said he wants those implicated by Mr Mehlis' report to be tried by an international court.