Protest leader Suthep Thaugsuban is demanding political and electoral reforms before any vote is held and wants these to be overseen by a "people's council" that his movement will help nominate rather than by Yingluck, who is caretaker prime minister until the election, set for February 2.

Thailand's National Security Council said only 6,500 people gathered at the busy Asoke intersection in central Bangkok at around mid-day, although office workers and others lined the route of the march to voice support.

On December 9, when Yingluck called the election, about 160,000 protesters had massed around her office complex, and before that some had occupied ministries and other state buildings, but police say no more than 2,000 people are now camped out at the main protest sites in Bangkok's historic quarter. Demonstrators on Thursday held banners saying "We are anti-corruption" and "No elections before reform".

A separate group of about 1,000 student-led protesters marched to the US embassy. The United States has annoyed the protesters by calling for the democratic process to be respected, effectively endorsing the holding of an election. Nititorn Lamlua, a protest leader, said U.S. Ambassador Kristie Kenney ought to be transferred.

"If she needs to leave the embassy, she'll have to go by helicopter because she has badmouthed the protesters," he said.

Yingluck won a landslide victory in 2011 and her Puea Thai Party is well placed to win the next election because of Thaksin's enduring support in the populous north and northeast. Thaksin fled in 2008 before being sentenced to jail for abuse of power in a trial he says was politically motivated.

Suthep's movement gained impetus in early November after Yingluck's government tried to push through a political amnesty bill that would have allowed Thaksin to return home a free man.