Most of the dead were killed by troops and militia loyal to President Bashar al-Assad and most of the missing are believed to have been detained by the government's secret police and other loyalists, the monitoring group said.
"The vast majority of civilian victims were killed by the regime. Killings in unofficial jails are commonplace, and the conditions under which prisoners are held are horrific," said Rami Abdulrahman, the Observatory's president.
The Observatory, established by Abdulrahman in Britain seven years ago, said 4,788 children were among the 34,473 civilians killed. Another 12,916 anti-Assad fighters were killed, along with 1,924 army deserters, it said.
On the loyalist side, 16,729 troops and 12,000 militiamen and informers have been killed. The report said the fate of around 2,500 loyalist troops believed to be held by rebels is unknown.
"The figures we have are the documented killings" Abdulrahman said. "We estimate the total number of people killed since the revolt to have surpassed 120,000."
(Reporting by Khaled Yacoub Oweis; Editing by Jon Hemming)