Robert Mugabe has resigned as Zimbabwe's president, shortly after parliament began an impeachment process to end his nearly four decades of rule.

The 93-year old clung on for a week after an army takeover and expulsion from his own ruling ZANU-PF party, which also told him to leave power.

Wild celebrations broke out at a joint sitting of parliament when Speaker Jacob Mudenda announced Mr Mugabe's resignation and suspended the impeachment procedure.

Thousands of Zimbabweans poured onto the streets of the capital Harare in celebration after Mr Mugabe's resignation. 

Mr Mugabe made the resignation in a letter submitted to parliament. 

In the letter, Mr Mugabe said that his decision to resign was made voluntary on his part. He also said that he has resigned to allow the smooth transfer of power.

The letter did not make any mention of who is now in charge of Zimbabwe. However, Mr Mudenda said that legal issues are being worked on to make sure a new leader is in place by the end of tomorrow. 

The origin of Mr Mugabe's sudden downfall lies in rivalry between members of Zimbabwe's ruling elite over who will succeed him, rather than popular protests against his rule.

The army seized power after Mr Mugabe sacked ZANU-PF's favourite to succeed him, Emmerson Mnangagwa, to smooth a path to the presidency for his wife Grace, 52, known to her critics as "Gucci Grace" for her reputed fondness for luxury shopping.

Mr Mnangagwa, a former security chief known as 'The Crocodile', is expected to take over as president.

Earlier, the parliament began an impeachment process against Mr Mugabe.

Mr Mudenda told parliament earlier that he received a motion to impeach and lawmakers would adjourn to a hotel to start the proceedings later today.

Thousands of people demonstrated outside parliament urging his resignation.

Since the crisis began, Mr Mugabe has been mainly confined to his lavish "Blue Roof" residence in the capital.

Mr Mugabe led Zimbabwe's liberation war and is hailed as one of post-colonial Africa's founding fathers and a staunch supporter of the drive to free neighbouring South Africa from apartheid in 1994.

But many say he has damaged Zimbabwe's economy, democracy and judiciary by staying in power for too long and has used violence to crush perceived political opponents.

The country faces a foreign exchange payments crisis and roaring inflation.

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Mr Mugabe held his weekly cabinet meeting this morning but only five ministers and the attorney general turned up. 17 others opted to attend a meeting to plan the impeachment.

"The people of Zimbabwe have spoken with one voice and it is my appeal to President Mugabe that he should take heed of this clarion call ... to resign so that the country can move forward and preserve his legacy," Mr Mnangagwa said in a statement.

Reuters reported in September that Mr Mnangagwa was plotting to succeed Mr Mugabe with army backing at the helm of a broad coalition.

The plot posited an interim unity government with international blessing to allow for Zimbabwe's re-engagement with the world. Its main aim was to stabilise its economy.

The former vice president was a key lieutenant to Mr Mugabe for decades and stands accused of participating in repression against Zimbabweans who challenged the leader.

Mr Mnangagwa said he has fled Zimbabwe because of a threat to his life after being purged from the ruling party.

He said he had been in contact with Mr Mugabe and invited to return but would not do so until his security could be guaranteed.

Mr Mnangagwa said his desire was to join all Zimbabweans in a "new era" that would rebuild the economy for the benefit of all.