Javed Hashmi is campaigning for the return of civilian rule

However, Mr Hashmi will effectively serve at most seven years in jail as he was handed seven different prison terms which will run concurrently.

He was arrested last year over a letter critical of President Pervez Musharraf.

An aide insisted Mr Hashmi was innocent and announced an appeal, saying the courts were in "Musharraf's clutches".

The verdict has proved that judiciary is in Musharraf's clutches Siddiqul Farooq PML-N spokesman

The BBC's Paul Anderson reports from Islamabad that the opposition leader's detention has been heavily criticised by opposition politicians and human rights organisations at home and abroad as a sign Pakistan's military rulers will not tolerate dissent.

Mr Hashmi is the leader of the Alliance for the Restoration of Democracy, a body campaigning for the return of civilian rule after Gen Musharraf's seizure of power in 1999.

He is also acting leader of the PML-N, the Pakistan Muslim League faction still loyal to former prime minister Nawaz Sharif who was ousted in the coup.

'Darkest chapter'

Javed Hashmi was arrested after circulating a letter bearing a military letterhead which was purportedly written by disgruntled officers.

It called for an inquiry into alleged corruption in the army's senior ranks and demanded a judicial investigation into a Pakistani military operation in Indian-administered Kashmir in 1999.

Pakistan's army has dominated the country's affairs

The authorities claimed the letter, which was also highly critical of President Musharraf and his alliance with the United States, was a forgery.

Mr Hashmi's allies say they believe the letter was genuine and that the charges of forgery are politically motivated.

He was convicted at a trial behind closed doors in the city of Rawalpindi, near Islamabad.

Special Public Prosecutor Munir Bhatti told Reuters news agency that he had been convicted on seven counts and, in addition to receiving the prison sentence, was fined 42,000 rupees ($720).

The verdict marked "the darkest chapter in the judicial history of Pakistan", said Siddiqul Farooq, a PML-N spokesman.

"The verdict has proved that judiciary is in Musharraf's clutches," he told AFP news agency.