Critics of outgoing Thai PM vow to disrupt by-polls

Thaksin's critics want the 4.2 million voters, mostly in southern provinces, to tear up their ballot papers, wear black clothing or choose the "none of the above" option in a poll they say lacks legitimacy.

The by-elections are being held after the inconclusive April 2 poll Thaksin called three years early to end weeks of street protests against him. The three main opposition parties boycotted that poll.

Sunday's polls are crucial to pulling Thailand out of a months-long political crisis as parliament cannot convene to name a new prime minister until all 500 seats are filled.