Tanks have been deployed on the streets of the Thai capital, Bangkok, after Prime Minister Abhisit Vejjajiva declared a state of emergency amid growing lawlessness.
But soon there was chaos, as protesters with distinctive red shirts worn by supporters of former Prime Minister Thaksin Shinawatra ignored the announcement, overrunning armoured vehicles.
Protesters also took over the interior ministry earlier, forcing Mr Abhisit to flee after delivering a televised address.
The protesters - who are demanding Mr Abhisit's resignation - overpowered the Thai police easily, seizing their weapons.
They attacked a car they thought was carrying Mr Abhisit, but the Thai leader was not believed to have been in it.
The prime minister threatened "tough measures" to end the protests, and hospitals in the city have been put on alert.
Sunday's violence comes a day after protesters forced the cancellation of a major Asian summit in the seaside resort of Pattaya.
Then too, protesters overcame Thai security forces, taking over the venue for the summit.
Delegates were evacuated by helicopter due to security concerns, in a situation that was seen as highly embarrassing to Prime Minister Abhisit.