Thailand has been rocked by years of often-violent demonstrations by rival protest movements.
The latest rallies, led by senior opposition figures, seek to curb the political dominance of Prime Minister Yingluck Shinawatra's older brother Thaksin, a controversial tycoon turned politician who was ousted as premier in a military coup in 2006.
The protesters want an unelected "people's council" to run the country and oversee vaguely defined reforms to tackle corruption and alleged vote buying before new elections are held in around a year to 18 months.
They say Yingluck is a puppet of her brother who controls her government from overseas, where he fled in 2008 to avoid going to jail for a corruption conviction. The rallies were triggered by a failed amnesty bill which could have allowed Thaksin to return without going to prison.