Anti-government group a disparate collection of democrats and royalists backed by urban middle class.
Thaksin, who was ousted from power in a 2006 military coup and recently convicted in absentia of breaching a conflict of interest law, is expected to address the crowd on Saturday evening.
Selina Downes, reporting from Bangkok for Al Jazeera, said the arena was full to capacity, with others waiting outside the stadium for Thaksin to deliver his address.
The former leader’s speech comes just two days before Thailand’s parliament votes on a new prime minister.
Thaksin faces two years in jail if he returns to the country from self-imposed exile.
Thaksin’s political opponents have threatened renewed street action if parliament chooses a leader they find unacceptable.
Somchai Wongsawat, Thaksin’s brother-in-law, was forced to step down as prime minister 10 days ago after a court found his People’s Power Party (PPP) guilty of fraud in the December 2007 election.
The court ordered that the PPP and two other parties in the coalition be disbanded, although all three have since regrouped under new names.
“The military and the Democrats are launching a disguised coup, claiming to have royal backing and coercing our friends to form a Democrat-led government,” Jatuporn Prompan, one of the organisers of Saturday’s rally, said.
Thaksin is seeking to rally support for the latest incarnation of his party, now named Phuea Thai, which is comprised mostly of politicians from the disbanded PPP and its predecessor, the Thai Rak Thai party.
Phuea Thai wants Pracha Promnok, a former national police chief and leader of the allied Puea Pandin party, to be elected as the new prime minister.
But the opposition Democrat Party insists that it has enough support from ministers who have defected from the former ruling party to elect its candidate, Abhisit Vejjajiva.
The new leader will be Thailand’s fifth prime minister in a little more than two years.
Our correspondent said: “[Thaksin] is a hugely popular figure here, especially among the rural poor. People that we have spoken to here are angry about the way in which the government was dissolved by the court.
“However, the opposition democrats seem confident they can form a government … it does appear that some MPs previously loyal to Thaksin Shinawatra may have, in fact, crossed the floor… something that we wouldn’t have expected even a few weeks ago. That will not go down well with those supporters gathered here.”
The country has been disrupted by months of protests by an anti-Thaksin alliance that occupied the prime minister’s office from late August until last week and brought the capital’s airports to a standstill.