* Anti-government demonstrators resume street protests in Bangkok
* PM Yingluck has until March 31 to defend herself against rice charges
* Government supporters plan big rally on April 5
* Growing fears Thailand heading towards serious civil unrest (Adds details of scuffle near anti-corruption offices)
By Amy Sawitta Lefevre
BANGKOK, March 24 (Reuters) - Anti-government demonstrators in Thailand resumed street protests on Monday after lying low for weeks, piling pressure on increasingly beleaguered Prime Minister Yingluck Shinawatra, who is expected to face impeachment within days.
Her opponents were emboldened by a Constitutional Court decision on Friday to nullify last month's election, delaying the formation of a new administration and leaving Yingluck in charge of a caretaker government with limited powers.
Yingluck's opponents first took to the streets in late November. Twenty-three people were killed and hundreds wounded in the political violence before the protests began to subside earlier this month. But the court ruling appears to have given her foes a second wind.
The protests are the latest instalment of an eight-year political battle broadly pitting the Bangkok middle class and royalist establishment against the mostly rural supporters of Yingluck and her billionaire brother, former premier Thaksin Shinawatra, who was ousted in a 2006 coup.
There are growing fears that Thailand could be heading towards serious civil unrest. After months of restraint, Thaksin's "red shirt" supporters have begun making militant noises under hardline new leaders.
They plan a big rally on April 5, possibly in Bangkok, and the political atmosphere is expected to become even more highly charged in coming days.
Yingluck has until March 31 to defend herself before the National Anti-Corruption Commission (NACC) for dereliction of duty over a ruinous rice-buying scheme that has run up huge losses.
If the commission recommends her impeachment, she could be removed from office by the upper house Senate, which is likely to have an anti-Thaksin majority after an election for half its members on March 30.
In a sign of the potential trouble ahead, one hundred red shirts blocked entrances to the NACC's offices in north Bangkok with sandbags on Monday to prevent officials there from working as police formed a wall to stop the group from facing off with anti-government protesters gathered nearby.
Earlier, red shirt supporters attacked a Buddhist monk, slightly injuring him, near the NACC offices after he insulted them for blocking a road in front of the complex.
The Constitutional Court annulment of the election could offer a way out of the political stalemate if the main opposition Democrat Party, which boycotted the Feb. 2 poll, decides to run in a fresh vote. So far, however, the Democrat Party has given no clear indication on what it plans to do.
The Election Commission, which is in charge of organising the new poll, met on Monday to decide how to proceed. Its chairman has said it would take at least three months to organise a new vote once a date is agreed.
It is increasingly uncertain whether Yingluck will last that long, due to the mounting legal challenges.
The prospect of her removal has bolstered the confidence of protest leaders.
Protest leader Suthep Thaugsuban has said he will lead a march every day this week to urge supporters to join a "massive" rally in Bangkok on Saturday to press for political reforms before a new vote takes place.
"Our rally will be the biggest signal to Yingluck Shinawatra and the Thaksin regime that the Thai public does not want elections before reforms," Suthep said in a speech on Sunday.
His supporters prevented voting in 28 constituencies on Feb. 2, providing grounds for the Constitutional Court to annul the election. Yingluck's supporters say the court, set up after the 2006 coup that removed her brother, has a record of ruling against parties linked to the former premier.
At the height of the protests more than 200,000 people took to the streets to demand Yingluck's resignation and to try to rid the country of the influence of Thaksin, whom they accuse of nepotism.
The protesters want an unelected "people's council" installed to oversee electoral changes that would, among other things, prevent close Thaksin allies from running for office.
(Additional reporting by Aukkarapon Niyomyat and Chaiwat Subprasom; Editing by Alan Raybould and Simon Cameron-Moore)