Yingluck arrived at the National Anti-Corruption Commission (NACC) in Bangkok on Monday but made no comment to the media as she entered the building or as she left 10 minutes later, an AFP reporter said.

She was summoned to answer charges linked to a controversial rice subsidy scheme, which paid farmers above market rates for their crops.

Observers say that after months of street protests, the kingdom's political crisis is lurching towards a critical new phase, with the NACC appearing set to move against the embattled premier.

"The prime minister gave verbal and written testimony... she asked the NACC to question 10 more witnesses and give more time for her lawyers to submit more evidence," commission member Prasart Pongsivapai told reporters.

"We will discuss both of these requests tomorrow... we are glad that she came, the atmosphere was good," he added.

If indicted by the anti-graft agency, the prime minister would be immediately suspended from office pending an impeachment vote in the upper house of parliament within weeks.

But Prasart refused to be drawn on a possible timeframe for the NAAC's response and denied accusations that the commission had sped up the process to assist the anti-government movement on Bangkok's streets.

Critics say the rice scheme battered Thai finances and fostered massive corruption, simply to shore up the rural base of Yingluck and her brother, the divisive self-exiled billionaire Thaksin Shinawatra who was ousted as premier in a 2006 coup.

The issue has become a lightning rod for Yingluck's political opponents who have massed on Bangkok's streets for months in a bid to topple her government.

Yingluck has protested her innocence, but if she is found guilty faces a possible five-year ban from politics, as well as imprisonment by the courts on criminal charges.

- Political limbo -

The kingdom, riven by a festering eight-year political division, is now trapped in legislative limbo with only a caretaker government following incomplete elections in February that were later annulled by the Constitutional Court.

While the nine-strong NACC panel is an independent body, government supporters say it is politically biased against the administration.

They accuse the NACC of failing to complete a near four-year-old graft case against former premier Abhisit Vejjajiva, while a welter of graft cases against Suthep Thaugsuban -- the firebrand leader of the anti-government street movement -- also remain unheard.

In recent days Thailand's upper house Senate, which would preside over any impeachment proceedings, has moved onto centre-stage of the political drama.

Polls for the elected portion of the Senate -- representing a narrow majority of the 150-seat upper house -- were held on Sunday.

The rest of the house is appointed by institutions seen as being allied to the anti-government establishment, including the Constitutional Court and Election Commission.

But as senators do not formally represent political parties it unclear how the vote would affect the balance of power in parliament.

Anti-government protesters appear to have pinned their hopes of ousting Yingluck on the kingdom's legal agencies and the upper house.

"The NACC is an independent body... but I'm sure it will provide justice over the case," their spokesman Akanat Promphan told AFP.

Thailand has been bitterly split since 2006 when Thaksin, a tycoon-turned-politician who lives in Dubai to avoid prison for a corruption conviction, was ousted as prime minister by a military coup.

- Reds ready to defend PM -

Pro-government "Red Shirts" have vowed to fight any attempt to remove Yingluck from office, promising a show of force on Saturday after weeks of relative quiet.

Red Shirts have vowed not to spark any violence but several hardliners have told AFP they are preparing military-style training for volunteers in the event of Yingluck being ousted.

"We have practised strategies... If there is a coup there is likely to be bloodshed," one militant Red Shirt leader from a province in northeastern Isaan told AFP, requesting anonymity.

The Red Shirts' street rallies against the previous government in 2010 resulted in bloody street clashes and a military crackdown that left dozens dead.

Political violence, often targeting protesters, has seen 23 people killed and hundreds wounded in grenade attacks and shootings in recent months, although the bloodshed has eased since rallies were scaled back at the start of March.