* Unclear when new election may be held
* Anti-government protesters say they will disrupt vote again
* Protesters want PM out, electoral reforms (Recasts after court decision)
By Amy Sawitta Lefevre
BANGKOK, March 21 (Reuters) - Thailand's Constitutional Court on Friday nullified February's election that was disrupted by protesters, further delaying the formation of a new government after months of street protests aimed at bringing down Prime Minister Yingluck Shinawatra.
In a 6 to 3 vote, the court ruled that the election violated the constitution, as voting had not taken place in 28 districts in southern Thailand where candidates were unable to register due to blockades by the anti-government protesters.
"The Feb. 2 election could not take place across the kingdom on the same day," the court said in a statement.
It was unclear when a new election would take place. Somchai Srisutthiyakorn, an Election Commission member, said the country had two options to organise a new vote.
"The commission could discuss with the government about issuing a new royal decree for a new date or we could ask the heads of all political parties to decide together when best to set the new election date," he told reporters.
If a new election is organised, the protesters say they will disrupt it again.
"If the court rules the election void, don't even dream that there will be another election. If a new election date is declared, then we'll take care of every province and the election won't be successful again," protest leader Suthep Thaugsuban told supporters late on Thursday.
The protests are the latest chapter in an eight-year crisis that pits Bangkok's middle class and royalist establishment against supporters of Prime Minister Yingluck Shinawatra and her brother, ex-premier Thaksin Shinawatra, who was toppled by the army in 2006 and lives in exile to avoid a jail term for graft.
Now in their fifth month, the protesters have shut government offices and at times blocked major thoroughfares in Bangkok to try to force Yingluck out. Twenty-three people have died and hundreds have been injured in the violence.
RISK OF MORE VIOLENCE
Yingluck called the election in December to try to defuse the protests and since then has headed a caretaker government with limited powers. The violence and political paralysis has dented confidence, prompting cuts to economic growth forecasts.
Yingluck's Puea Thai Party had been expected to win but the main opposition Democrat Party threw in its lot with the protesters and has demanded electoral changes before any vote, seeking to reduce the influence of Thaksin. Parties led by or allied to him have won every election since 2001.
The protesters retreated this month to a Bangkok park and the battleground has moved from the streets to the courts. Yingluck faces a spate of legal challenges that could bring down her government, including a charge of dereliction of duty related to a disastrous rice-buying scheme.
Thaksin's "red shirt" supporters, who are strong in the north and northeast, are beginning to make militant noises, raising the prospect of more violence if Yingluck is forced out by the courts, the anti-corruption commission or by other means.
"Independent agencies are being quite obvious that they want to remove her and her entire cabinet to create a power vacuum, claim that elections can't be held and then nominate a prime minister of their choice," said Kan Yuenyong, a political analyst at the Siam Intelligence Unit.
"If they run with this plan, then the government's supporters will fight back and the next half of the year will be much worse than what we saw in the first half," he said.
The streets have been relatively calm since several big protest sites were shut at the start of March and a state of emergency was lifted on Wednesday.
However, police reported that three grenades exploded just before midnight on Thursday near the home of one of the Constitutional Court judges. One person was slightly injured. (Additional reporting by; Aukkarapon Niyomyat and Apornrath Phoonphongphiphat; Writing by Alan Raybould and Amy Sawitta Lefevre; Editing by Michael Perry)