A bomb blast struck an anti-government protest march in the Thai capital on Friday, wounding at least 28 people, officials said, sending tensions soaring following weeks of mass opposition rallies.
It is the latest in a series of attacks by unknown assailants against demonstrators seeking to oust Prime Minister Yingluck Shinawatra.
The protest movement said the blast happened shortly before rally leader Suthep Thaugsuban was due to march past the spot.
"The bomb went off about 30 metres (100 feet) from Suthep and then his bodyguards escorted him back to a rally stage," spokesman Akanat Promphan told AFP.
Television footage showed several people lying on the ground as ambulances rushed away the wounded. Protesters were seen searching nearby buildings for the attackers.
Police were investigating what type of explosive device caused the blast.
An official from the city's Erawan emergency centre said 28 people were hurt in the explosion, without giving details of the injuries.
Yingluck has faced more than two months of street demonstrations aimed at forcing her elected government from office and installing an appointed body to oversee loosely defined reforms such as an end to alleged vote buying.
Eight people have been killed and hundreds injured in street violence since the protests began.
There have been a series of drive-by shootings at rally sites and grenade attacks on the houses of opposition politicians that both the demonstrators and the government have blamed on each other.
"Yingluck must take responsibility," one of the protest leaders, Satit Wonghnongtaey, said on stage soon after the blast.
"This government, Yingluck and Red Shirt thugs are creating violence," he said, referring to a rival pro-government protest movement whose rallies in 2010 were suppressed in a bloody military crackdown.
The government denied the claim, saying the protesters were trying to incite violence.
"A movement has been set up to create a situation of bomb attacks against leaders' houses and protesters," Deputy Prime Minister Surapong Tovichakchaikul told reporters.
The opposition demonstrations aim to curb the political dominance of Yingluck's billionaire brother, fugitive former premier Thaksin Shinawatra, whom they accuse of controlling the government from self-exile.
On Thursday the National Anti-Corruption Commission launched an investigation into possible negligence of duty by Yingluck in connection with a controversial subsidy scheme for rice farmers.
Yingluck has called an election for February 2 in an effort to defuse the kingdom's deepening political crisis but the main opposition Democrat Party is boycotting the polls, which they fear will only return the Shinawatra family to power.
Demonstrators have occupied major intersections in the capital since Monday in what they have dubbed the "Bangkok shutdown".
The government has urged police to detain rally leader Suthep, who faces an insurrection charge -- in theory punishable by death -- in connection with the protests.
The protests were triggered by a failed amnesty bill that could have allowed Thaksin to return without going to jail for a past corruption conviction.
The billionaire tycoon-turned-politician has strong electoral support in northern Thailand, but he is reviled by many southerners, Bangkok's middle class and members of the royalist establishment.