Zimbabwe's parliament opened a session to begin impeachment proceedings against President Robert Mugabe on Tuesday as ousted vice president Emmerson Mnangagwa, who could be the country's next leader, told him to step down.

Zanu PF, the ruling party, tabled a no-confidence motion urging parliament to remove Mr Mugabe from office for a string of offenses including falling asleep in meetings and allowing his wife to "usurp" presidential powers. 

"We have seen the president sleeping in Cabinet and international meetings to the horror, shame and consternation of Zimbabweans," reads the motion, which was seconded by the parliamentary leader of the Movement for Democratic Change, the main opposition party. 

Mr Mugabe also is accused of allowing Grace Mugabe, the first lade, to threaten to kill Mr Mnangagwa. 

A joint session of both houses of parliament must now appoint a committee to investigate the claims and report back on whether or not a vote of no confidence will follow. 

Further street protests have been called in Harare, raising fears that the political turmoil could spill over into violence.

Mr Mugabe also suffered humiliation on Tuesday when almost no government ministers heeded his call to attend a cabinet meeting at his State House residence, official media reported.

The snub piled pressure on the embattled president after Mr Mnangagwa, the vice president whose removal by Mr Mugabe sparked the military intervention last week, said he would consider returning to Zimbabwe if his safety was guaranteed.

Mr Mnangagwa's intervention is his first public move since the army seized control.

Lawmakers of Mr Mugabe's once-loyal Zanu-PF party met in parliament at 12.15pm (GMT) to trigger proceedings that could see the president stripped of office.

Parliament speaker Jacob Mubenda gave permission for a joint session of the House of Assembly and the Senate to debate a motion that would trigger impeachment proceedings against Mugabe.

"This motion is unprecedented in the history of post-independence Zimbabwe," Mr Mubenda declared.

Dozens of protesters gathered near parliament, chanting for Mr Mugabe to resign and brandishing Zimbabwean flags and banners emblazoned with "Mugabe go".

A bubbling factional squabble over the presidential succession erupted two weeks ago when Mr Mugabe fired Mr Mnangagwa.

The dismissal put Mr Mugabe's wife Grace in prime position to succeed her 93-year-old husband, prompting the military to step in to block her path to the presidency.

After Mr Mnangagwa fled abroad, the army took over the country and placed Mugabe under house arrest - provoking amazement and delight among many Zimbabweans as his autocratic 37-year reign appeared close to an end.

Mr Mnangagwa - formerly one of Mugabe's closest allies and a regime hardliner - said in his statement that Zimbabweans had "clearly demonstrated without violence their insatiable desire" for Mugabe to resign.

"It is my appeal to President Mugabe that he should take heed of this clarion call," he said.

The influential war veterans' association appeared to pull back from an earlier call for immediate demonstrations at Mugabe's home, instead threatening further protest action if Mr Mugabe clung on.