Sutin, a core leader of the People's Democratic Force to Overthrow Thaksinism, was shot dead in a clash outside a polling station in Bang Na, where protesters had gathered to block yesterday's advance vote in the general election.

The main vote is set for Sunday.

Speaking at the Pathumwan intersection stage hours after the fatal shooting, Mr Suthep urged the military to form a special unit to help safeguard the protesters.

"The soldiers may set up checkpoints around rally sites to search for weapons and provide protection, because we don't trust the police," Mr Suthep said.

He said he wondered if the shooting was the work of Cambodia's special warfare unit, which he believes has been brought in to Thailand by senior police figures to attack the protesters.

"The rights and freedoms of the people are guaranteed under the constitution, so we hope the armed forces see this fighting and will protect us," he said.

However, he said he was not calling for a military coup.

Mr Suthep criticised the Centre for Maintaining Peace and Order (CMPO), which has invoked an emergency decree in Bangkok to deal with the protest situation.

He said it was unable to enforce its own laws, and this allowed Sutin to be killed in the heart of the city.

PDRC spokesman Ekanat Prompan said as the attack took place during the invocation of the emergency decree, caretaker Prime Minister Yingluck Shinawatra and caretaker Labour Minister Chalerm Yubamrung, who oversees the CMPO, must take responsibility for it.

Sutin, 52, was shot dead at Wat Sri Iam in the Bang Na area, and another 10 people were injured in the polling station attack at 1.45pm yesterday, according to the Narenthorn Emergency Centre of the Public Health Ministry.

They were taken to nearby hospitals, but Sutin was pronounced dead from wounds to the head, neck and right chest at Vibharam Hospital. The injured survivors are in a stable condition.

Sutin led a group of protesters to the polling station at Wat Sri Iam about 11.30am after learning that booths were open for advance voting.

Somnuek Chaisiri, 47, who was among the protesters, said they went to close down the station.

Sutin and Rawee Mashmadol, another Pefot core leader, went to negotiate with the director of the polling station, who finally agreed to cease operations. The protesters then prepared to leave, Mr Somnuek said.

After the group had gone only a short distance, Mr Somnuek said he saw a group of people carrying red flags and waiting for the protesters.

The protesters tried to avoid them, but at least five gunshots were fired from among this group.

"I saw Sutin and three other people being shot [while standing] on a pickup truck," he said.

Meanwhile Pol Lt Col Kunthon Prachuapmoh, a Bang Na constituency MP candidate for the Pheu Thai Party, denied leading the group to the station to create violence, following reports that he was among them during the chaos at Wat Sri Iam yesterday.

"I had no reason to get involved," he said. "My two subordinates and I went out to see the situation but had not yet reached the scene [when the shots were fired]."

Shortly after the attack, a foreigner by the name of Roy Nijland uploaded a video clip called "Suthin ying dtaai [Sutin shot dead] _ From Supalai park Srinakarin" on to YouTube.

In the 2:57-minute clip, which is believed to have been taken from the Supalai Park Srinakarin condominium, protesters are seen outside the polling station. Two gunshots and screams are then heard. Men then rush at passengers on a pickup truck and damage the vehicle until a woman shouts "Enough!".

Dr Rawee said he will lodge a complaint with police against those who killed Sutin and injured others.