The original ruling had marked the first time a former leader of the armed forces had faced such a sentence for treason in Pakistan, where the military maintains strong influence and senior officers are often considered immune from prosecution.

It caused a wave of controversy, with Musharraf -- exiled in Dubai -- slamming it as a "vendetta" and the military expressing its disappointment.

A High Court in the eastern city of Lahore ruled it "illegal" on Monday.

"The filing of the complaint, the constitution of the court, the selection of the prosecution team are illegal, declared to be illegal... And at the end of the day the full judgment has been set aside," the prosecutor representing the government, Ishtiaq A. Khan, told AFP.

"Yes, he is a free man. Right now there is no judgment against him any longer," Khan added.

The prosecution now has the option to file a new case against Musharraf with the approval of the federal Cabinet.

The treason trial -- which began in 2013 and is just one of several involving Musharraf -- centred on his decision to suspend the constitution and impose emergency rule in 2007. The case went on for years amid repeated delays until the surprise announcement last year.

The imposition of emergency had sparked widespread protests against Musharraf, ultimately leading to his resignation in the face of impeachment proceedings.

Musharraf first took power after ousting prime minister Nawaz Sharif in a bloodless coup in 1999.

A cigar-smoking, whisky-drinking moderate, the general became a key US ally in the "war on terror" after the September 11 attacks and escaped at least three Al-Qaeda assassination attempts during his nine years in office.