The 27th amendment, passed by a two-thirds majority, also consolidates military power under a new Chief of Defence Forces role and establishes a Federal Constitutional Court.

The changes grant army chief Asim Munir, recently promoted to field marshal after Pakistan's clash with India in May, command over the army, air force and the navy.

Munir, like other top military brass, would enjoy lifelong protections.

Any officer promoted to field marshal, marshal of the air force, or admiral of the fleet will now retain rank and privileges for life, remain in uniform, and enjoy immunity from criminal proceedings -- protections previously reserved only for the head of state.

"This constitutional amendment will increase authoritarianism and whatever little semblance of democracy existed in this country will fade away," said Osama Malik, an Islamabad-based lawyer.

"It will not only remove civilian oversight from the military's activities, it will also completely destroy the military hierarchy where all service chiefs were considered equal under the joint chief system," he told AFP.

The amendment also ensures immunity for President Asif Ali Zardari, shielding him from any criminal prosecution.

However, the bill specifies that this protection would not apply if a former president later holds another public office.

Zardari has faced multiple graft cases, although proceedings were previously stayed. The latest amendment makes him untouchable unless he takes on another public office.

The amendment also bars courts from questioning any constitutional change "on any ground whatsoever".

The bill now heads to the president to be signed into law.