By Aung Hla Tung

YANGON, September 9 (Reuters) - Myanmar's generals accused detained opposition leader Aung San Suu Kyi's National League for Democracy on Sunday of a political attack on the state through a string of protests and threatened it with unspecified action.

State television broke into a Sunday afternoon movie to make the announcement, which could lead to the NLD, the landslide winner of 1990 elections the military nullified, being declared illegal after protests against huge fuel price rises last month.

"The NLD took advantage of the increase of fuel prices by the state and tried to exploit the situation to mount a political attack," the surprise announcement said a day after the junta accused 13 jailed dissidents of terrorism.

"They send letters to international organizations, embassies and governments, requesting assistance," it said in another indication the junta is determined to squash the dissent that has mushroomed since the shock fuel prices increases.

"They cooperated with the so-called '88 Generation Student Group' and exile groups to bring about demonstrations, riots and terrorist acts similar to 1988," it said referring to an uprising the army put down with the estimated loss of 3,000 lives.

"The government will never tolerate such malicious acts and will take effective action against those committing them."

NLD spokesman Nyan Win denied the party had done anything illegal.

"I don't think they have enough reasons to take legal action against the NLD because our party does not commit these things," he said.

MORE LONG JAIL TERMS

The junta's accusation of terrorism against 13 detained dissidents, most of them leaders of the 88 Generation Student Group who spent long years in jail after the uprising, reinforced expectations they would be jailed again.

They were rounded up in midnight raids shortly they organised a protest against a doubling of diesel prices and a five-fold rise of compressed natural gas prices and accused of crimes carrying prison terms of up to 20 years.

"The terrorists will be exposed and legal action will be taken against them," the former Burma's ruling generals said on Saturday in another rare announcement on radio and television.

Min Ko Naing, the most prominent dissident after Suu Kyi, is among the 13. Suu Kyi, the Nobel laureate who has spent 12 years under some form of detention, is isolated under house arrest without a telephone and needing permission to receive visitors.

That denunciation, accompanied by a long account of the group's alleged subversion, came a day after six people who ran a labour rights seminar at the American Center in Yangon were sentenced to at least 20 years for plotting against the state.

Four of the labour activists received 20 years for sedition, five years under "illegal association" laws, and three years for immigration offences, lawyer Aung Thein said.

The other two were found guilty only of sedition and given a 20-year jail term plus a fine of 1,000 kyats -- about 75 U.S. cents.

"What they did at the May Day ceremony was explain labour rights to the workers," Aung Thein, who said he was forced to quit as the group's attorney by police harassment, told Reuters. "It had nothing to do with sedition."

Families were allowed into the court, but the accused had no defence lawyer.

Continuing its hunt for a handful of prominent activists still at large, official papers also call on the public to keep their eyes open for "saboteurs", saying the government, people and army must unite to crush "the enemies within and without".

The crackdown, one of the harshest since 1988, has drawn withering criticism from the United States and European Union, and unusually strong words from Myanmar's Asian neighbours.

Even China, the generals' main trading partner and the closest they have to a friend, said it wanted to see "reconciliation and improvement in the situation". ($1=1,350 kyats)

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