The Thai government said on Tuesday it was extending a state of emergency in Bangkok and three surrounding provinces for three more months, in the wake of deadly street protests in April and May.

The state of emergency was prolonged "in four provinces including Bangkok," deputy government spokesman Suphachai Jaismut said.

The controversial decree was introduced in the capital in early April in response to mass anti-government rallies by the "Red Shirt" movement that ultimately left 91 people dead as protesters and the army clashed.

The laws ban public gatherings of more than five people and give security forces the right to detain suspects for 30 days without charge.

The government has stepped up security again following a string of grenade attacks in Bangkok, including a blast at a bus stop in July that killed a man.

The Red Shirts deny any involvement in the explosions and have accused the government of a conspiracy to justify tougher security powers.

The decision to extend the decree was taken in the absence of Thai Prime Minister Abhisit Vejjajiva, who left his deputy Suthep Thaugsuban in charge while he attends an Asia-Europe summit in Brussels this week.

The Thai government has come under pressure from the United States and rights groups to roll back the emergency powers to help the country recover from civil violence that has left it deeply divided.

The two-month rally by the Reds, many of whom back fugitive ex-premier Thaksin Shinawatra, attracted up to 100,000 people demanding immediate elections, but was broken up by an army crackdown on May 19.

After the crackdown, enraged protesters set dozens of major buildings ablaze in the heart of Bangkok, including a shopping mall and the stock exchange.

Authorities have used their emergency powers to arrest hundreds of suspects and silence anti-government media.

Rights group Amnesty International said last week the rules "flout international human rights law and standards".

"As a result, the extraordinary powers they grant to curtail human rights have often been abused to block the expression of peaceful dissenting views," it said.

The cabinet also appointed Defense Minister General Prawit Wongsuwon as head of the government unit which oversees the use of the emergency rules, replacing deputy premier Suthep, who is resigning to run for parliament.