BBC reporter Rachel Harvey: "I get the sense the government has had enough"
The Thai military says security forces plan to surround a protest camp in Bangkok with armoured vehicles.
The military gave a deadline of 1800 (1100 GMT) for people to leave the area. Shops and businesses were urged to close and transport suspended.
As the deadline passed there was no sign of the armoured vehicles but trucks unloaded soldiers near the camp.
PM Abhisit Vejjajiva is under severe pressure to end the protests, which have paralysed Bangkok since 14 March.
The protesters, known as the red-shirts, want Mr Abhisit to dissolve parliament and call fresh elections.
He had offered polls on 14 November - but the two sides failed to agree a deal because of divisions over who should be held accountable for a deadly crackdown on protests last month.
The BBC's Rachel Harvey in Bangkok says the fear is that more blood might be spilled before this crisis is brought to an end.
The move comes a day after the government announced and then cancelled a plan to cut off water and power supplies to the protesters.
The military urged businesses around the protest camp to close by the early evening deadline and stay shut.
AT THE SCENE
Vaudine England, BBC News, Bangkok
This government deadline appears to be more serious than earlier ones.
We are now seeing covered trucks unloading new, combat-ready troops under a flyover a few blocks east of the red-shirt protest camp - but no armoured vehicles yet. Colleagues have seen many more in various parts of town, including scores of troops inside the large Lumphini Park which backs on to the red-shirt camp.
Sources have told us that many more troops have been inconspicuously gathering around the city in recent days.
Local TV is reporting that several key roads have now been cleared of traffic around the red-shirt camp. Public transport has been crammed with people trying to leave the area, exacerbating the usual rush-hour jams.
The red-shirts are making their own preparations - fiery speeches, barricade reinforcements, testing of generators and water pipes.
Anything - and even nothing - could happen in what has become an all-too-normal state of suspense.
"The authorities will seal off the protest area at all routes at 6pm today with armoured personnel vehicles. No-one would be allowed in," a spokesman, Colonel Sansern Kaewkumnerd, said.
He said that sharpshooters armed with live ammunition would move into position in the area.
A BBC reporter saw trucks unloading heavily-armed soldiers several blocks from the protest encampment.
Transport officials said four elevated Skytrain stations in and around the protest zone would close at 1800, and two subway stations. It is not clear whether water and power supplies will be cut.
The camp stretches from Bangkok's shopping hub south to the business district. Protesters have built large barricades from tyres and bamboo, behind which they have stockpiled food supplies and generators.
The military's announcement comes as hopes of a political solution to the crisis fade.
Mr Abhisit told journalists that he had withdrawn his offer of early elections in November.
"I have cancelled the election date... because protesters refuse to disperse," he said. "I have told security officials to restore normalcy as soon as possible."
A few days ago a deal had appeared within reach. But protest leaders demanded that charges be laid against the Deputy Prime Minister Suthep Thaugsuban over the 10 April crackdown.
14 Mar: Red-shirts converge on Bangkok, hold first big rally, occupy government district
16 Mar: Protesters splash their own blood at Government House
30 Mar: A round of talks with the government ends in deadlock
3 Apr: Red-shirts occupy Bangkok shopping district
10 Apr: Troops try to clear protesters; 25 people are killed and hundreds injured
22 Apr: Grenade blasts kill one and injure 85 near protest hub; each side blames the other
3 May: PM Abhisit offers reconciliation plan and polls on 14 November
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