Thaksin was due to address the crowd by video-link and organisers said the rally would move to the house of Prem Tinsulanonda, who the red shirts accuse of masterminding the coup, and who advises the country's king.
About 9,000 soldiers and police and imposed were deployed by the government in the city and an Internal Security Act imposed around the rally venue, allowing the military to close roads and make arrests.
Abhisit said there were reports that unidentified groups of troublemakers could set off bombs in the capital to create unrest.
"I am worried about the situation tonight and have warned intelligence agencies," he said.
From early morning on Saturday, military checkpoints had been set up as well as barricades to contain any potential violence.
King Bhumibol Adulyadej, 81, has reigned for more than six decades, serving as head of state through 15 successful or attempted coups and 16 constitutions.
Thai police deployed about 600 police to protect the house of Prem, the senior advisor to King Adulyadej.
Panitan Wattanayagorn, a spokesman for the Thai government, told Al Jazeera: "Demonstrations in a democratic society is normal as long as there is order, are held under the law and there is no violence."
Violent street protests by Thaksin's supporters and opponents have blocked roads, airports and government buildings and brought Bangkok to a halt twice in the past year.
Thaksin won landslide election victories in 2001 and 2005 but was overthrown in the coup.
But he was convicted last year by the supreme court's Criminal Division for Holders of Political Positions on conflict of interest charges in relation to a land purchase scandal.
Thaksin remains popular among Thailand's rural poor for the populist policies he introduced.
Despite being in self-imposed exile, he remains an influential force and has sent almost daily video and audio messages to Red Shirt rallies, supporting their call for the current prime minister to resign and hold fresh elections.
The Red Shirts took their cue from protests last year by rival Yellow Shirts, who took to the streets in huge demonstrations against successive pro-Thaksin governments.