* Army says declares martial law to restore stability
* Says not interfering with caretaker government
* To avoid clashes, orders protesters on both sides not to march
* TV stations run by rival political groups ordered to shut down
* U.S. says monitoring situation closely (Adds comment from army chief, adds detail)
By Amy Sawitta Lefevre
BANGKOK, May 20 (Reuters) - Thailand's army declared martial law nationwide on Tuesday to restore order after six months of street protests that have left the country without a proper functioning government, but insisted the surprise intervention was not a military coup.
While troops patrolled parts of Bangkok and army spokesmen took to the airwaves, the caretaker government led by supporters of self-exiled former premier Thaksin Shinawatra said it was still running the country.
Army chief General Prayuth Chan-ocha said the military had stepped in to restore order and build investor confidence, and warned that troops would take action against anyone who used weapons and harmed civilians.
"We ask all sides to come and talk to find a way out for the country," Prayuth told reporters after meeting directors of government agencies and other high-ranking officials.
Military officials said they were not interfering with the caretaker government, but ministers were not informed of the army's plan before an announcement on television at 3 a.m. (2000 GMT on Monday) and Prayuth said martial law would be maintained until peace and order had been restored.
Twenty-eight people have been killed and 700 injured since the anti-government protests began in November last year.
The crisis is the latest instalment of a near-decade-long power struggle between former telecoms tycoon Thaksin and the royalist establishment that has brought the country to the brink of recession and even raised fears of civil war.
Both pro- and anti-government protesters are camped out at different places in the capital and, to prevent clashes, the army told them they had to stay put and could not march.
The army also called on media not to broadcast material that would affect national security and ordered 10 satellite TV channels, including both pro- and anti-government stations, to stop broadcasting.
The caretaker government, wary of the army given its past interventions on the side of the establishment, said it welcomed the move to restore order and that it remained in office.
"The army's actions must be under the framework of the constitution," caretaker Prime Minister Niwatthamrong Boonsongphaisan said in a statement.
Thailand has been stuck in political limbo since Prime Minister Yingluck Shinawatra, Thaksin's younger sister, and nine of her ministers were dismissed on May 7 after a court found them guilty of abuse of power.
The military, which put down a pro-Thaksin protest movement in 2010, has staged numerous coups since Thailand became a constitutional monarchy in 1932. The last one was in 2006 to oust Thaksin, who has lived abroad since 2008 but wields political influence and commands huge support among the poor.
Anti-government protesters want a "neutral" prime minister appointed to oversee electoral reforms aimed at ending Thaksin's influence. The government views an early general election it would likely win as the best way forward.
Thaksin said anyone following political developments could have expected martial law.
"I hope that no side will violate human rights and damage the democratic process more than it has already been," he said in a rare message posted on his Twitter account (@ThaksinLive).
The army tried to mediate in the crisis late last year, bringing together then premier Yingluck and anti-government protest leader Suthep Thaugsuban. It has played down fears of a coup, stressing that politicians must resolve the dispute.
But Human Rights Watch called Tuesday's army action a "de facto coup" while a political analyst said it was a "phantom coup".
"There was no consultation with the government and I think the military will slowly expand its powers and test the waters," said Kan Yuenyong at the Siam Intelligence Unit think-tank.
"For this to be a success the army needs to act like a neutral force and not be seen to side with the anti-government protesters. It needs to offer an election date and start a political reform process at the same time."
Martial law gives the military broad powers over civilian authorities, but a full coup would likely incur costs in terms of greater damage to investor confidence and U.S. sanctions.
The United States, which cut aid to its military ally after the 2006 coup, said it was monitoring the situation closely.
"We expect the army to honour its commitment to make this a temporary action to prevent violence, and to not undermine democratic institutions," U.S. State Department spokeswoman Jen Psaki said in a statement.
Prayuth had warned last week, after three people were killed in an attack on anti-government protesters, that troops might have to be used if violence continued.
"The army chief was moving towards imposition of martial law ever since his announcement last week," said a senior army official, who spoke on condition of anonymity. "He now feels that the police cannot handle security and is alarmed by grenade attacks and other incidents and the fact neither side looks like it will back down."
Troops initially stopped some traffic from entering Bangkok, took up positions at some intersections and secured television stations but life went on as normal in most of the city.
"NO COUP YET"
The baht fell against the dollar in early trade but steadied later and dealers suspected that was due to intervention by the central bank. At 0900 GMT the baht was quoted at 32.53 per dollar after earlier trading at a low of around 32.64.
The stock market was down 1.1 percent in late trade.
Six months of turmoil has dragged down Southeast Asia's second-biggest economy, which shrank 2.1 percent in the first quarter of the year.
Andrew Colquhoun, Head of Asia-Pacific Sovereigns at ratings agency Fitch, said martial law was not necessarily negative for Thailand's government debt, and might help break the deadlock.
"The key factors for the ratings are whether Thailand can avert more serious and bloody political disorder, and whether we see a return to a fully functioning government that is able to make policy and pass a budget for the next fiscal year starting in October," he said.
Opposition supporters disrupted a Feb. 2 election which was later declared void by the Constitutional Court. The protesters reject any vote before electoral reforms and the Election Commission has said it does not think a poll tentatively scheduled for July 20 can go ahead.
The leader of Thaksin's pro-government "red shirt" loyalists, who are rallying in Bangkok's western outskirts, appealed for calm but warned of trouble if the government was ousted.
"Our stance is the same. (We) will not accept a neutral prime minister. If soldiers appoint a prime minister then we will escalate our rally," Jatuporn Prompan told a news conference. "Stay calm, there has been no coup yet."
Anti-government protesters said they too had not changed their demands for the caretaker government to go. (Reporting by Bangkok Bureau; Writing by Robert Birsel; Editing by Alan Raybould and Alex Richardson)