As Bangkok suffers its worst political unrest for two decades, the Thai army has declared parts of the city "live-fire zones", warning that anyone found entering certain roads in the capital will be shot on sight.

The move came as one of the leaders of the redshirt protesters said that there would be "civil war" if the army did not pull back and declare a ceasefire.

After two days of conflict, the army stepped up efforts to cut off the redshirts from the rest of the world, sealing off swaths of the city.

Troops have erected signs at Ratchaprarop Road, on the northern edge of the redshirts' camp, warning, in Thai and English: "No Entry, Restricted Area. Live Firing Zone". Protesters still risked their lives to harass troops, drawing fire as they ran across the deserted streets to hurl rocks and bottles. Others, including residents caught out by the army's sudden move, were seen fleeing the area with their hands above their heads.

Since violence re-erupted in Bangkok on Thursday with the shooting of renegade army general Seh Daeng, 24 people have been killed, bringing to 54 the death toll since 12 March. More than 1,100 have been injured, including more than 150 during the past three days.

International bodies called for calm as the UN secretary general, Ban Ki-moon, urged both sides to do everything in their power to avoid further loss of life. British ambassador Quinton Quayle and former US ambassador William Itoh have also called for the two sides to restart talks.

In the middle of the sprawling encampment in the heart of Bangkok's shopping and finance districts, redshirt leaders remained resolute. Weng Tojirakarn, head of the United Front for Democracy against Dictatorship, demanded the government declare a ceasefire and pull back its troops because "we don't want to see a civil war. If it does happen, I don't know how many years it will take to end."

Speaking publicly for the first time in two days, Thailand's prime minister, Abhisit Vejjajiva, said that his administration was acting in the interests of the majority of Thai people. "The government has no choice but to move forward [in ending the protest by force]," he said in an address carried live on every Thai TV channel.

"We have tried every other method to bring this to an end."

He asked that the people of Bangkok co-operate with the government's efforts and that protesters allow women and children to leave the camp.

On the streets, however, there is a growing realisation that the impasse will not end without further bloodshed. Behind the fortifications of bamboo staves and fuel-filled tyres, the redshirt guards say they are prepared to face the troops when they march in.

"We are all very afraid," Somchai Sanwong said as he manned the barricades, a few hundred yards from troop positions.

The redshirts have piles of rocks and Molotov cocktails stashed to hurl at troops when they finally advance. They also have gallons of motor vehicle oil to make the road slippery. Deeper inside the camp, sources say, the redshirts have dozens of M79 rocket-launched grenades. Several were fired last night at an inner-city police station.

"Obviously we're outgunned, outnumbered. In the worst case, if the soldiers come, we'll just burn the barricades," Somchai said.

In the no man's land between the redshirt fortifications and the troop roadblocks, some of the streets in Bangkok are eerily quiet. Businesses are shut and thousands of residents have fled.

The government says it has no timetable yet to order troops to march on the redshirts' barricades. It intends, for now, to restrict the protesters' communications and access to reinforcements, as well as supplies of food, water and power. After two days of porous army roadblocks that allowed supplies and thousands of extra protesters to reach the camp, movement in central Bangkok is now heavily restricted.

Vehicles and bags are being checked at roadblocks for supplies, and only those who can prove they are resident or work inside the cordoned-off area are allowed in.

Already there are reports of food shortages. By most estimates, the redshirts can only sustain themselves for a few more days.