A hospital in the Thai capital evacuated most of its patients yesterday after it was stormed by "Red Shirt" protesters who mistakenly believed troops were hiding there following recent violence.

Chulalongkorn Hospital also stopped receiving outpatients following the Thursday night incident, in which some 100 Red Shirt guards searched its buildings for troops they feared were preparing for a crackdown.

Tensions are high in Bangkok following the worst political violence in almost two decades which has left 27 people dead and almost 1,000 injured this month in a series of bloody confrontations.

The 1,400-bed hospital sent most of its patients to 10 other facilities in Bangkok and was considering how to cope with the protesters occupying a large part of the city's nearby commercial district, a spokesman said.

"For outpatients, if their symptoms are not serious we asked them to postpone treatment," the spokesman said.

The Reds, who have occupied sections of Bangkok for over a month in their bid to force snap elections, have alleged the hospital was used in a grenade attack on a pro-government rally last week that killed one and wounded dozens.

The government said the grenades were fired from inside the Reds' camp - an accusation the movement has denied.

The Reds publicly apologised for the storming of the hospital, saying guards told demonstration leaders that they saw soldiers inside the facilities, which are located near one of their main barricades.

"On behalf of all leaders, I apologise to the public and Chulalongkorn Hospital for the incident," Red leader Weng Tojirakarn said. "The situation got out of control.

"It is not our policy to obstruct hospital operations."

Thailand's Medical Council slammed the storming and asked protesters to respect medical personnel, while police said they would deploy 100 officers to the hospital to ensure neither security forces nor Reds use the grounds.

Many of the Reds come from Thailand's rural poor and urban working classes and seek the return of former prime minister Thaksin Shinawatra, who was ousted in a 2006 coup and now lives overseas to avoid a jail term for corruption.

Prime Minister Abhisit Vejjajiva vowed to prosecute those who were involved in the hospital incident.

"What happened last night was a shock to hospital management," he said in a television address. "Those who violated the law must be prosecuted."

After heavy public criticism of their raid, Reds scaled back barricades yesterday to give greater access to the hospital. The group said they also might remove barricades from an intersection in front of three major shopping malls.