HONG KONG, July 18 (AFP) - The government said Friday it would ban any protest deemed likely to advocate independence of Hong Kong, Tibet or Taiwan, in a move that pro-democracy activists lashed as going beyond tough political curbs introduced on July 1.

In a statement, the Special Administrative Region (SAR) government said it had issued guidelines to clarify the concept of "national security," introduced after Hong Kong's return to China, that determines whether protests can go ahead.

"The Commissioner of Police will invoke the national security ground only if he reasonably considers it necessary to do so," the statement quoted an SAR spokesman as saying.

"Among other things," the Commissioner should consider whether the demonstration's goal "is to advocate separation from the People's Republic of China, including advocacy of Taiwan or Tibet," it said.

"This falls under consideration of 'national security,' because such advocacy might threaten the territorial integrity and independence of the People's Republic of China," it contended.

Police could also intervene during the course of a protest if they consider that the meeting, or someone taking part in it, might cause "an imminent breach of the peace" or call for independence of Hong Kong, Taiwan and Tibet, it added.

Controls over public protests and political parties were tightened on the first day of China's resumption of sovereignty over Hong Kong.

The Beijing-created Provisional Legislature passed laws outlawing any demonstration that had not been authorised by the police, who would determine whether a rally might violate "national security" and other factors.

That term was strongly criticised by liberal politicians and human-rights groups as being too vague and vulnerable to a Draconian interpretation.

They renewed their attack on Friday, saying the long-awaited clarification had only served to tighten the government's grip on dissent.

"It will take away the basic rights of citizens in Hong Kong to demonstrations," said Lee Cheuk-yan, general secretary of the Hong Kong Confederation of Trade Unions.

"We believe it is very inappropriate for police to determine the political screening of demonstrations, because their role is to maintain order. Their role is not to control freedom of expression."

Margaret Ng, like Lee a former member of the elected Legislative Council that was ousted by the Provisional Legislature, said, "it is a blatant contravention of the freedom of speech."

"It is way beyond the ambit of even the Public Order (Amendment) Ordinance." she told RTHK radio.

"To say that, when a purpose of a certain meeting or procession is of a certain nature that sets off certain powers of the police to interfere, seems to be me to be very clearly wrong."

Analysts said the guidelines may stoke discontent with SAR Chief Executive Tung Chee-hwa, who is already seen by many as taking an authoritarian line.

But, they said, there was negligible risk of any confrontation on the independence issue.

Calls for independence of Hong Kong have never gained significant backing here. Nor has there been any visible support for self-rule in Tibet, which is recognised in international politics as being part of China, or for the independence of Taiwan, which Beijing considers a renegade province.