Nawaz Sharif is serving a seven-year prison term at the Kot Lakhpat Jail in Lahore. (Reuters file photo)

A senior Pakistani judge on Sunday denied being blackmailed into convicting ex-Prime Minister Nawaz Sharif for graft last year after the main opposition party showed secretly taped video that appeared to show him saying he was threatened over the case.

Nawaz Sharif's Pakistan Muslim League-Nawaz (PML-N) party presented the video at a specially called news conference on Saturday, saying it proved that Sharif, currently serving a seven-year sentence for corruption, had been wrongfully convicted and should be released.

In the secretly shot video, judge Arshad Malik, a senior judge on the accountability court, is seen telling a man described by PML-N officials as a party sympathiser that unidentified individuals had confronted him with compromising video footage to pressure him into convicting.

He appears to tell the man that "they called me to a place and put on a film on a TV and left the room. After three or four minutes later, they entered the room and said take it easy. After watching the movie, I thought of committing suicide."

Reuters has not been able to verify the authenticity of the video presented by the PML-N.

In a statement issued through the accountability court, Malik said that his comments in the video were pieced together and presented out of context and he had never faced intimidation to rule against Sharif.

"I want to make it clear that I was neither directly or indirectly intimidated, nor was I influenced by greed," he said, adding that he had faced pressure from the former prime minister's allies not to convict.

"During the hearing of the cases against Nawaz Sharif and his family, I was repeatedly offered bribes by their representatives and also given threats of serious consequences if I did not cooperate."

A PML-N spokeswoman said Malik did not deny the conversation recorded on tape had taken place and asked why, if he had faced pressure from Sharif's camp, he had not reported it previously.

"Why did he not report to the Supreme Court supervisory judge about threats, bribes and blackmail?" she said in messaged comments.

The incident highlighted the increasingly feverish political atmosphere in Pakistan, where the government of Prime Minister Imran Khan swept to power last year vowing to root out and punish corruption by past governments.

Since then, it has run into growing economic and political headwinds and was forced to turn to the International Monetary Fund for Pakistan's 13th bailout since the 1980s.

As the problems have mounted, opposition parties have stepped up attacks, accusing the government and its allies in the powerful military establishment of crushing dissent and orchestrating Sharif's removal from power and conviction.

Maryam Nawaz, Nawaz Sharif's daughter and vice president of the PML-N, said the video showed the judge had been intimidated into passing the conviction against her father.

"This evidence is enough to prove Nawaz Sharif is innocent and he should be released immediately," she said during Saturday's press conference.

Nawaz Sharif is currently serving a seven year sentence for being unable to prove the source of income that had led to his ownership of a steel mill in Saudi Arabia. Under Pakistani law, this is taken to prove corruption.

Pakistan information minister Fidous Ashiq Awan called the video an attack on the entire judiciary by the PML-N and said a forensic audit of the video would be conducted.

"The judge's conversation and the contents' credibility will be evaluated. Are they real or tampered?" she said.

Sharif, once a favourite of Pakistan's powerful generals before falling out with them, was ousted and disqualified from holding office by the Supreme Court in July 2017. He was convicted in absentia a year later and arrested on returning from London.

He denied the charges which he said were politically motivated and accused the military and courts of working together to end his political career. The military has denied interfering in civil or judicial matters.