Organisers have predicted that 600,000 Thaksin supporters and opponents to the 2006 coup that removed him from power will attend Sunday's march, although the government has put the figure at about 100,000.
The government has enacted the Internal Security Act to monitor the rally, allowing for checkpoints, curfews and limitations on movement.
"We are watching out for possible incidents. We don't want to exaggerate but small numbers of people may try to cause incidents ... like throwing hand grenades," Panitan Wattanayagorn, a government spokesman, said.
A rally stage was set up close to government buildings on Saturday. Most protesters are expected to arrive in Bangkok by Saturday evening.
Al Jazeera's Wayne Hay, in Bangkok, said: "The influx for Red Shirts (the colour worn by Thaksin supporters) has been slower than expected so numbers are lower that originally thought at the moment.
"Numbers of course will grow but will be well below the stated aim of one million."
Sean Boonpracong from the United Front for Democracy Against Dictatorship, responded by telling Al Jazeera that he thought the number of protesters would be large.
"From what we have monitored ourselves it is the opposite. We are on target we will have at least half a million.
"For us to achieve a very small objective for this government to dissolve the parliament we need a huge amount of people to show then that there is dissatisfaction
"That's out limited objective and we want to show the world that all is not well."
However, many businesses were closed in the city on Saturday and traffic was lighter than usual.
Thaksin, who currently lives in Dubai, is wanted on charges of abuse of power.
The red shirts called the protests two weeks after Thailand's highest court confiscated $1.4bn of Thaksin's assets, frozen after the coup.
The red shirts argue that the government of Abhisit Vejjajiva, the Thai prime minister, came to power illegitimately with the backing of coup leaders and the ruling elite, and they are demanding new elections.
Thaksin has been encouraging the protests via text messaging and the Internet.
"Thank you for your dedication ... I want to give my support to the people in the north," he told his supporters, mostly the rural poor, on Twitter on Saturday.
The demonstrations are set to be the biggest since the Red Shirts held protests last April, which descended into violence in which two people died.
The Red Shirts, who benefited from Thaksin's populist policies during his tenure, have said that Abhisit's government is elitist and ignores their democratic rights.
Opponents of the red shirts - the so-called yellow shirts - say that Thaksin is corrupt and fails to show sufficient loyalty to the revered Royal family.