Protesters accuse the billionaire businessman Thaksin of rampant graft and want to remove the influence of family, promising ill-defined political reforms.

The violence is the latest episode in an eight-year conflict that pits Bangkok's middle class and royalist establishment against poorer, mainly rural supporters of Thaksin and his sister.

The government has mostly avoided direct confrontation with protesters while the army, which has staged or attempted 18 coups in 81 years of on-off democracy, has stayed neutral.

The violence is the worst since 2010 when Suthep, at the time a deputy prime minister, sent in troops to end mass protests by pro-Thaksin supporters.

Suthep faces murder charges related to his role in the 2010 military crackdown when more than 90 people were killed, and for insurrection in leading the latest protests.

Yingluck faces legal challenges with the country's anti-corruption agency saying last week it would start investigating her role in loss-making government rice purchase scheme.

The scheme has won her party huge support in the rural north and northeast of the country. But there are signs of growing discontent among farmers who say they have not been paid for their rice and are threatening to block major roads.

Chambers said the rise in violence could suck the police into the fray.

"(That would provide) Suthep with an excuse to accuse Yingluck of repressing the demonstrators, the army may suggest that the Yingluck government step aside or judicial cases against Yingluck's government may be expedited to push (her party) Puea Thai from power," he said.