The order issued by the president on holding a referendum is valid

Pakistan Supreme Court

But thousands of opposition supporters held a demonstration against the referendum in the city of Lahore.

They are calling for a boycott, and also accused the government of deliberately withholding permission for their rally until the last moment to limit the turnout.

President Musharraf says he is seeking another five years in power in order to continue his programme of political and economic reforms.

The BBC's Zafar Abbas in Islamabad says the ruling is a major boost to General Musharraf in his efforts to convince people at home and abroad that the referendum is a lawful move.

Constitution

In its ruling, the Supreme Court said that the referendum did not violate the constitution as it had been suspended since the military takeover in 1999.

President Musharraf: Took over in military coup in 1999

The country is now governed by President Musharraf's provisional constitutional order, it said, so he was entitled to call the referendum.

"The grounds of the challenge... are purely academic, hypothetical in nature and are not capable of being determined at this juncture."

The court said a more detailed explanation would be issued at a later stage.

It is a very sad day in the constitutional history of Pakistan

Lawyer Farooq Hassan

Senior lawyers for the government praised the decision, but Farooq Hassan, a lawyer for one of the religious parties, the Jamaat-e-Islami, condemned the ruling.

"It has once again confirmed the suspicions that our judiciary does not have the strength to stand up... to an authoritarian executive," he said.

Under the constitution, Pakistan's president is normally elected by the national and provincial assemblies and the senate.

But General Musharraf says the constitution allows a referendum to be held on matters of national importance.

The Pakistani leader is obliged to hold general elections by October this year.

Observers say the referendum will allow him to consolidate his hold on power ahead of the planned elections.

General Musharraf's controversial referendum has united religious parties and the country's main political parties, who have called for a boycott.

The president is holding rallies around the country to garner support for extending his rule.