Those in the dock are accused of "treason" and trying to overthrow the head of state, President Teodoro Obiang Nguema, both charges which could bring the death penalty.
Further charges are "terrorism" and possession of arms and ammunition, which alone could bring 20-year jail terms.
In January of last year the government said it had foiled an attempted coup, which it said was organised by foreign mercenaries who had a month earlier tried to overthrow Obiang.
At the time he was in his palace at Koete Mongomo some 50 kilometres (30 miles) from the bordres with Gabon, Guinea and Cameroon.
On December 27, 2017, three days after the alleged operation, Cameroon police detained a group of armed men on the Cameroon border.
There followed dozens of arrests in Equatorial Guinea and warrants were issued against a number of foreigners believed to have been involved.
Only 79 of the 150 accused are in the dock for what is expected to be a three-week trial -- including 24 foreigners, mainly from Chad and the Central African Republic.
The defendants' lawyers demanded their clients' immediate release for lack of evidence. They also cited procedural "irregularities" in the case.
Those in the dock include several former regime supporters including former ambassador to Chad, Henri Nsue Anguesomo, as well as the former head of presidential security Julian Ondo Nkumu, an AFP journalist said.
Around 30 suspected foreign mercenaries have not been extradited from Cameroon, where they were arrested after the alleged coup.
They included British-based exiled opposition figure Salomon Abeso Ndong and five Frenchmen.
Lawyers for those absent have urged their cases be closed.
The prosecution earlier showed the court what it says is material evidence of the coup planning including around 20 shotguns with ammunition and satellite telephones.
Equatorial Guinea is one of Africa's top oil producers but the bulk of the population of some 1.2 million lives in poverty. The country has lived through several coups since independence from Spain in 1968.
Nguema, 76, is now Africa's longest-serving leader having himself seized power in a 1979 putsch.
Critics accuse him of brutal repression, election fraud and corruption.