Anti-government protesters marched in Bangkok in a bid to force Prime Minister Yingluck Shinawatra from office but their numbers appeared far smaller than earlier in the month.

Ms Yingluck called a snap election to try to defuse the crisis after huge street protests at the beginning of December.

Protest leader Suthep Thaugsuban is demanding political and electoral reforms before any vote is held.

He wants these to be overseen by a "people's council" that his movement will help nominate rather than by Ms Yingluck, who is caretaker primeminister until the election, set for 2 February.

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

A separate group of about 1,000 student-led protesters marched to the United States embassy.

The US 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 US 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.

On 9 December, when Ms Yingluck called the election, about 160,000 protesters had massed around her office complex.

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 held banners saying "We are anti-corruption" and "No elections before reform".

One sign read: "We will not accept Square Face", a nickname given to former prime minister Thaksin Shinawatra, Ms Yingluck's brother and the figure at the centre of Thailand's eight-year, on-off political crisis.

Mr Thaksin, a former telecommunications tycoon, is adored by the rural poor because of cheap healthcare and other policies brought in while he was in power, but he was toppled by the military in 2006 and now lives in self-exile.

Ms Yingluck won a landslide victory in 2011 and her Puea Thai Party is well placed to win the next election because of Mr Thaksin's enduring support in the populous north and northeast.

Ranged against them are a royalist establishment that feels threatened by Mr Thaksin's rise and a middle class that resents what it sees as its taxes being spent on wasteful populist policies that amount to vote-buying.

Mr Thaksin fled in 2008 before being sentenced to jail for abuse of power in a trial he says was politically motivated.

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

After failing to get the politically influential military on his side, Mr Suthep is trying to re-energise his supporters with marches this week and a rally on Sunday.

Arrest warrant issued for protest leader

A court has issued an arrest warrant for Mr Suthep on a charge of insurrection but police have done nothing to apprehend him, despite his appearance at a military seminar and other events.

Yesterday the Department of Special Investigation, Thailand's equivalent of the US FBI, said it would ask banks to freeze the accounts of 18 rally leaders, including Mr Suthep, to investigate what it called suspicious transactions.

Mr Suthep says he wants to wipe out electoral fraud, eradicate corruption and reform state agencies including the police.

Even if the vote goes ahead on 2 February, its legitimacy could be undermined if the main opposition Democrat Party does not take part.

At a conference this week, members could not agree whether to run in the election or back the protests.              

Democrat lawmakers resigned from parliament this month to march with Mr Suthep, who was a deputy prime minister in a Democrat-led government until 2011.

Some agree with his call for reforms to be implemented before another election is held, but others believe their party, Thailand's oldest, should respect the democratic process and run for office.

A decision is expected on Saturday.