Hospital sources said at least 36 people had been injured in the blast that hit demonstrators taking part in an anti-government protest in central Bangkok on Friday. None of the injuries were thought to be life-threatening.

Thailand's deputy national police spokesman, Anucha Romyanan, said the explosive device had gone off shortly after 1 p.m. (0600 UTC) local time and that an investigation had been launched.

This is just the latest incident of violence related to the anti-government demonstrations, which began two months ago, after Yingluck's government attempted to pass amnesty legislation that would have paved the way for a return of her brother, former Prime Minister Thaksin Shinawatra.

A total of eight people have been killed and hundreds of others wounded in street violence related to the anti-government demonstrations, which Yingluck's opponents have blamed on the authorities. It was not immediately clear who was behind Friday's attack.

In the latest protest action, which began on Monday, demonstrators have been occupying main roads and junctions in the Thai capital, in a series of rallies meant to cause major disruption to government and businesses.

Late last year, the prime minister called snap elections for February as a way of resolving the political crisis, but this failed to satisfy the demonstrators, who are demanding that Yingluck step down immediately to make way for the appointment of an unelected committee to draw up democratic reforms.

Thailand has been hit by repeated unrest since the country's army ousted Thaksin Shinawatra in 2006.

He has been living in exile since 2008 to avoid serving time in prison after being convicted on corruption charges.

pfd/kms (Reuters, AFP, dpa)