* Protesters hold first major rally in weeks
* Senate election could help to remove PM Yingluck
* Yingluck to submit defence against rice charges on Monday (Adds details, quotes)
By Amy Sawitta Lefevre
BANGKOK, March 29 (Reuters) - Tens of thousands of Thai anti-government protesters rallied across Bangkok on Saturday in their latest bid to topple Prime Minister Yingluck Shinawatra, a day before a crucial vote to elect a new Senate.
Waving flags and blowing whistles, protesters marched from Lumpini Park in the business district of Bangkok, where protesters retreated to earlier this month, toward the city's old quarter after a brief hiatus in anti-government rallies.
"The rally has been largely peaceful and very disciplined. Protesters are now heading back to their base in the park after a series of symbolic ceremonies," Paradorn Pattanathabutr, a security adviser to the prime minister, told Reuters.
"We expected the crowd to be around 50,000-strong but the number of protesters doesn't look like it will exceed 30,000."
A grenade exploded as protesters passed the Foreign Ministry offices, but no one was hurt, police said. It was unclear who was responsible for the attack.
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.
Saturday's march is seen as a test of the anti-government movement's popularity as the number of protesters has dwindled considerably in recent weeks.
By mid-afternoon police put the crowd at around 20,000. Around 500 protesters from the Network of Students and People for the Reform of Thailand, a splinter group of the main protest group, broke into the compound of Government House, a site largely abandoned by officials.
Over the past five months, protesters have shut state offices and disrupted a Feb. 2 election which was nullified by a court on March 21, leaving Thailand in political limbo and Yingluck at the head of a caretaker government with limited powers.
Election officials have said it will take at least three months to organise a new election.
Since the current round of protests kicked off in November, 23 people have been killed in sporadic political violence.
Protesters want political and electoral reforms before a new general election and to rid the country of Thaksin's influence.
"We will no longer accept this oppressive regime. They, Thaksin and Yingluck, are no longer welcome in Thailand," protest leader Suthep Thaugsuban told reporters as he led protesters who shouted "Yingluck, get out!".
Yingluck has dismissed calls by protesters to step down but faces several legal challenges that could lead to her removal. She has until Monday to defend herself before the National Anti-Corruption Commission (NACC) for dereliction of duty over a 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 may have an anti-Thaksin majority after an election for half its members on Sunday.
The vote is to elect 77 senators for the 150-seat Senate. The rest are appointed, and a government attempt to make it a fully elected body was one of the sparks that set off the latest unrest in November.
The non-elected Senators are picked by judges and senior officials from agencies such as the National Anti-Corruption Commission (NACC), members of an establishment whom government supporters see as viscerally anti-Thaksin.
"Red shirt" supporters of Yinguck and Thaksin are sounding more militant under hardline new leaders and say they are prepared to take to the streets of Bangkok as moves to impeach Yingluck gather pace, increasing the risk of a confrontation.
They plan a big rally, possibly in Bangkok, on April 5.
At the height of the current protests more than 200,000 people took to the streets to demand Yingluck's resignation.
(Additional reporting by Chaiwat Subprasom; Editing by Jeremy Laurence)