A Thai anti-corruption agency has said it will investigate a money-guzzling rice subsidy programme that has fuelled opposition to Prime Minister Yingluck Shinawatra.

It comes as protesters marched through the capital Bangkok demanding she resign.

The protesters want Ms Yingluck to make way for an unelected "people's council" that would oversee reforms to curb the political dominance of her billionaire family.

The unrest flared in November and escalated this week when demonstrators led by former opposition politician Suthep Thaugsuban occupied main intersections of Bangkok.

However, the number of people camping out overnight at some of the intersections appears to be dropping.

The turmoil is the latest episode in an eight-year conflict that pits Bangkok's middle class and royalist establishment against the mostly poorer, rural supporters of Ms Yingluck and her brother, former premier Thaksin Shinawatra.

Ms Yingluck's Puea Thai Party was helped to power in 2011 by offering to buy rice at way above the market price to help poor farmers.

Critics say the programme is riddled with corruption and that it has cost taxpayers as much as 425 billion baht (€9.5bn), although that figure would drop if the government managed to find buyers for the rice in state stockpiles.

"Those who oversaw the scheme knew there were losses but did not put a stop to it," Vicha Mahakhun, of the National Anti-Corruption Commission (NACC), told a news conference.

Ms Yingluck is nominally head of the National Rice Committee and could therefore eventually face charges.

The intervention price made Thai grain so expensive Thailand lost its position as the world's top rice exporter, overtaken by India and Vietnam.

Mr Thaksin's rural and working-class support has ensured he or his allies have won every election since 2001 and Puea Thai seems certain to win an early election Ms Yingluck has called for February.

The anti-government protesters have rejected the election. They want to suspend what they say is a fragile democracy destabilised by Mr Thaksin, whom they accuse of nepotism and corruption.

Their goal is to eradicate the political influence of his family by altering electoral arrangements, though in ways they have not spelt out, along with other political reforms.

In a separate ruling, the NACC said it had grave doubts about government-to-government deals announced by former Commerce Minister Boonsong Teriyapirome.

He and other officials will now be summoned to explain themselves and the NACC will then decide whether to file formal charges.

Many ministries and state agencies have closed to avoid violence, with staff working from home or back-up facilities.

The protesters are trying to paralyse ministries, marching each day from camps they have set up at the seven intersections.

They targeted revenue offices today.

Earlier, Thailand's government pleaded with police to arrest opposition protest leaders who threatened to take the prime minister captive and have paralysed parts of central Bangkok.

Leaders of the anti-government movement still travel freely around the city delivering fiery speeches and collecting money from supporters, despite warrants for their arrest for their roles in civil unrest that has left eight dead and hundreds injured.

Rally leader Mr Thaugsuban faces an insurrection charge, theoretically punishable by death, in connection with the protests.

He also faces a murder charge linked to a military crackdown on opposition protests that left dozens dead when he was deputy premier in 2010.

"It's the duty of the police to arrest Suthep because he is wanted for insurrection, otherwise police will face malfeasance charges," Deputy Prime Minister Surapong Tovichakchaikul said after a meeting with the national police chief.

Mr Surapong said Mr Suthep, a former opposition MP, was protected by about 40 personal bodyguards.