Most of the dead - members of the People's Temple Christian Church - had consumed a soft drink laced with cyanide and sedatives.
However, the body of the People's Temple charismatic leader, Jim Jones, was said to have a bullet wound in the right temple, believed to be self-inflicted.
The deaths are being linked to the earlier killings of five people, including US Congressman Leo Ryan, on a nearby airstrip.
Mr Ryan had led a fact-finding mission to the church's jungle settlement - Jonestown - after allegations by relatives in the US of human rights abuses.
Last year Jim Jones and most of the 1,000 members of the People's Temple moved to Guyana from San Francisco after an investigation began into the church for tax evasion.
People who had left the organisation told the authorities of brutal beatings, murders and a mass suicide plan but were not believed.
In spite of the tax evasion allegations, Jim Jones was still widely respected for setting up a racially-mixed church which helped the disadvantaged.
Five dead at airport
Leo Ryan's delegation arrived in Jonestown on 14 November and spent three days interviewing residents.
They left hurriedly earlier on Saturday after an attempt on Mr Ryan's life, taking with them about 20 People's Temple members who wished to leave.
Delegation members told police as they were boarding planes at the airstrip a truckload of Jim Jones' guards arrived and began to shoot.
When the gunmen left five people were dead: Congressman Ryan, a reporter and cameraman from NBC, a newspaper photographer and one "defector" from the People's Temple.
A producer for NBC News, Bob Flick, survived the attack.
Mr Flick said: "Every time someone fell down wounded they would walk over and shoot them in the head with a shotgun."