Alistair Leithead reports on the military presence in Bangkok's business district
The Thai army has moved hundreds of soldiers into the business district of Bangkok to prevent anti-government protesters entering the area.
The protests by the "red-shirts", now in their fifth week, are currently focused in the city's shopping hub.
The red-shirts are trying to force Prime Minister Abhisit Vejjajiva to step down and call an election.
They had said they might move to Bangkok's financial district but later said they would not do so.
The government's supporters, known as the yellow-shirts, have vowed to hold their own protests if security forces do not clear the red-shirts within seven days.
Government figures said the aim of moving troops into place in Bangkok was not to attack the protesters, but simply to block any attempt they might make to enter the area.
AT THE SCENE
By Rachel Harvey, BBC News, Bangkok
There's a slightly surreal feel to all this today. Some of the protesters are manning a barricade looking out across the intersection. On the other side of that intersection are some soldiers with assault rifles.
Every few 100 metres there are either riot police or soldiers and some razor wire that's been set up across the pavement and some military trucks.
But the traffic is flowing freely down the middle of the road and there are people going about their business. The shops are all open. There are some rather bemused looking tourists that are having to make their way around the razor wire. And on the upper walkways, I can see lots of people up there, just curious onlookers, taking photographs.
The music in the background is being played by the soldiers ... to try and calm the atmosphere. Every now and then people bring up cold drinks to the soldiers and the police. It is ferociously hot here today and they're standing in their full gear in the heat of the day.
Everybody is going about their normal daily lives but with this significantly increased militarised presence on the streets.
Late on Friday, Mr Abhisit put the army commander-in-chief, Anupong Paojinda, in charge of national security.
The general held a meeting of military commanders on Sunday, issued warnings to the red-shirt protesters to avoid the business district, and moved his troops into the area before dawn on Monday.
Soldiers are on patrol in the city's famous red light district of Patpong, and along Silom Road, where the headquarters of Bangkok Bank is located along with many other office buildings.
The bank is a target of the protesters' ire due to its establishment links. Head of the Privy Council and former prime minister Gen Prem Tinsulanonda is an adviser to the bank.
"There are several units currently armed to prevent themselves from attacks from terrorists who are hiding among protesters," said government spokesman Panitan Wattanayagorn.
The government uses the word "terrorist" to refer to an alleged militant hard-core of the anti-government movement, which has been largely peaceful.
The army spokesman, Sunsern Kaewkumnerd, said that the troops - in their first deployment since last weekend's failed crackdown - were carrying live ammunition.
"The authorities have their right to protect themselves," Mr Sunsern told AFP.
An army spokesman told the BBC that plans were being formulated to bring the current crisis to an end. But, he said, any action would be proportionate.
The BBC's South East Asia correspondent, Rachel Harvey, says that after weeks of reacting to the protesters' changing tactics, Thailand's security forces seem to have decided that pre-emptive moves might be more effective.
On Sunday, leaders of Thailand's yellow-shirt movement, which supports the political establishment, gave the government a week to end the red-shirts' protests or warned they would hold their own demonstrations.
The Peoples' Alliance for Democracy (PAD), the formal umbrella for the yellow-shirts, had last week called for martial law.
Formally called the United Front for Democracy Against Dictatorship (UDD)
Mostly poorer workers from rural areas
Many are loyal to ousted PM Thaksin Shinawatra
Believe Mr Abhisit came to power illegally and want him to resign and call elections
Known as the Peoples' Alliance for Democracy
Loose coalition of mostly urban middle-class royalists and businessmen
United by their hatred of Mr Thaksin who was ousted in 2006
Occupied airports and official buildings in 2008, precipitating a political crisis
Some members had expressed frustration that the attempted military crackdown of 10 April failed to end the red-shirt protests. Twenty-five people were killed and more than 800 wounded.
The group has been largely silent since it closed down the country's airports in December 2008 and helped usher in the current government.
Other much smaller groups, claiming to be "no colour", have also held gatherings in recent days, calling for an end to the political confrontation.
Despite calls from some hard-line parts of the establishment, Gen Anupong has advocated a political solution and said his troops were not planning further crackdowns.
The army plays a prominent role in Thai politics - former Prime Minister Thaksin Shinawatra was ousted by the military in 2006. Many of the red-shirt protesters want Mr Thaksin to return.
In a telephone interview with Reuters news agency, Mr Thaksin said the prime minister had to call a snap election in order to end the stand-off.
Chavalit Yongchaiyudh, a former prime minister and head of the main opposition Thaksin-allied Puea Thai party, said he was seeking an audience with the Thai king to ask him to intervene to end the stand-off.
The stock market began falling after the 10 April bloodshed, going down 6.8% last week and continuing its fall by another 1.7% by midday on Monday.
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