Anti-government protesters wave flags as they follow their leader Suthep Thaugsuban marching in downtown Bangkok March 27, 2014. REUTERS/Damir Sagolj

By Panarat Thepgumpanat

BANGKOK (Reuters) - Grenades were thrown at the offices of Thailand's anti-corruption agency, which has summoned the prime minister to answer charges of dereliction of duty next week, as protesters trying to oust her prepared for a big weekend rally.

Nobody was injured in the overnight attack, the second on the agency's offices this week, police said on Friday. It was not clear who threw the grenades. Supporters of Prime Minister Yingluck Shinawatra have been demonstrating at the building this week.

The National Anti-Corruption Commission is examining the alleged failure of Yingluck to stop corruption and stem huge losses in a government rice-buying programme.

It is widely expected to recommend her impeachment by the Senate. If the Senate takes up the case, Yingluck will have to step aside, with a deputy prime minister expected to take over. If she is found guilty by the Senate, she has to step down.

Her supporters, who have been restrained during five months of anti-government protests in the capital, are starting to mobilise and are planning their own big rally, or series of rallies, on April 5.

Anti-government demonstrators resumed street protests on Monday after lying low for weeks. Their rally on Saturday is expected to draw up to 50,000 people Paradorn Pattanathabutr, a security adviser to the prime minister, told Reuters.

"We don't expect any violence at the rally but provocateurs might try to stir trouble to discredit the government side," Paradorn said.

Thailand has been in crisis since former premier Thaksin Shinawatra, Yingluck's brother, was ousted in a 2006 coup. The conflict broadly pits the Bangkok-based middle class and royalist establishment against the mostly poorer, rural supporters of the Shinawatras.

The turmoil entered a fresh phase in November, when anti-government protesters first took to the streets, with 23 people killed during the political violence over the following months

The protesters disrupted a general election on February 2 and the ballot was annulled by the Constitutional Court this month.

Yingluck heads a caretaker administration with limited powers, unable to take any big policy decisions binding on the next government.

The political paralysis is hurting the economy.

Data on Friday showed factory output fell 4.42 percent in February from a year before, the 11th fall in a row.

And although customs data on Wednesday logged a 2.4 percent rise in exports in February compared with a year before, imports plunged 16.6 percent after a 15.5 percent drop in January, showing the weakness of domestic demand and the reluctance of industry to invest in capital goods because of the crisis.

(Additional reporting by Amy Sawitta Lefevre; Writing by Alan Raybould)