Two people have died in what appears to have been a grenade attack at an anti-government protest site in Thailand's capital.

Those killed in Sunday's attack in central Bangkok were a 40-year-old woman and a 12-year-old boy, the Erawan emergency medical services centre said. The centre says 22 other people were injured.

Six protesters were hurt Friday night by a grenade attack in the same area.

On Saturday night, gunmen in a pickup truck attacked an anti-government protest in eastern Thailand, killing a 5-year-old girl and wounding dozens of other people, as violence in the country's 3-month-old political crisis spread outside the capital.

The attacks were the latest in a spate of protest-related violence roiling Thailand over the past three months, with at least 16 people killed and hundreds hurt. The protesters want Prime Minister Yingluck Shinawatra to quit to make way for an appointed interim government to implement anti-corruption reforms, but she has refused.

While the protesters have failed repeatedly to force Yingluck out through self-declared deadlines, they have blocked the prime minister from working at her normal offices and have sent roving mobs after her, making it difficult for her and Cabinet colleagues to make public appearances. The protesters also have succeeded in delaying completion of an early election called by Yingluck, undermining efforts to restore political stability.

Saturday night's attack took place in Trat province, where about 500 anti-government protesters were rallying near busy food stalls. Trat is about 300 kilometres east of Bangkok.

A nurse at Trat Hospital, Nantiya Thientawatchai, said a 5-year-old girl died in the attack.

Police Lt. Thanabhum Newanit said unidentified assailants in a pickup truck shot into the crowd and two explosive devices went off. It was not clear whether the protest group's guards fought back. He and other officials said that about three dozen people were hurt, several critically.

'Massacre of innocents'

Both supporters and opponents of the protesters, as well as police, have been victims of the political violence, which before Saturday was mostly confined to Bangkok.

A spokesman for the protesters, Akanat Promphan, described the attack in Trat as "a massacre of innocents" that was "planned and organized terror."

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Thai police officers inspect the site of an explosion late Saturday in the Khao Saming district of Trat province. (Reuters)

"The authorities must quickly find those terrorists responsible. Yingluck must show responsibility. Otherwise, we can only assume the government and ... Yingluck's involvement in this atrocity," he said.

Both sides in the ongoing political dispute have blamed the other for instigating violence.

"At this point we do not know who was behind the attack, but there are several factors to take into account in the investigation," said National Security Council chief Lt. Gen. Paradorn Pattanathuabutr.

He said the protesters in Trat have been rallying for a long time, "so they might have caused disturbance to others," adding that the "area is controlled by groups that are affiliated with the anti-government side."

Thailand has seen sometimes violent political conflict since 2006, when then-Prime Minister Thaksin Shinawatra, Yingluck's brother, was ousted by a military coup after being accused of corruption and abuse of power. Thaksin's supporters and opponents have since taken to the streets for extended periods in a power struggle.

In 2010, pro-Thaksin "Red Shirts" occupied part of Bangkok for two months, and were assisted by their own armed militia. More than 90 people were killed in violent confrontations, with the army finally ordered to sweep away the demonstrators.

The Red Shirts have mostly kept a low profile during the current political unrest, but held a meeting Sunday in the northeastern province of Nakhon Ratchasima to discuss how to respond to the political crisis.