Zimbabwe's parliament has begun formal proceedings to impeach Robert Mugabe, with its speaker announcing to the house that a motion to remove the President from office has been brought forward by the war veterans leader Chris Mutsvangwa and seconded by an opposition politician.

The full motion was expected to accuse Mr Mugabe of allowing his wife to "usurp" his position, and of being too old to effectively carry out his duties.

As politicians arrived at parliament on Tuesday afternoon to begin the impeachment proceedings, thousands of Zimbabweans attended a rally in the capital Harare in support of the motion and the actions of the military in placing Mr Mugabe under house arrest.

"We are here because we want to be part of this very important occasion in the history of this country," said Harare resident Samuel Wadzai.

Zimbabwe opposition leader Morgan Tsvangirai told the crowd outside Parliament that a "democratic Zimbabwe cannot be built by another un-democratic process."

Mr Tsvangirai said the culture of the ruling party "must end" and everyone must put their heads together and work toward free and fair elections.

"Now the question is, how do we end Mugabe," said the opposition leader, who shared power with Mr Mugabe as prime minister for a number of years.

It is not clear how long impeachment would take, though the ruling party has said it could vote Mr Mugabe out as early as Wednesday.

Earlier, in a bid to show he was still in control of the country, Mr Mugabe called a meeting of his Cabinet.

But the last gasp attempt to stay in power appears to have backfired. According to the Reuters news agency, only five ministers and the Attorney General answered Mr Mugabe's call, with the 17 other members of Cabinet instead attending a meeting to plan the 93-year-old leader's impeachment.

Lovemore Matuke, a senior official of the ruling Zanu-PF party told the Associated Press that "all" ministers snubbed the Cabinet meeting and instead heeded a directive to attend the party caucus.

Four regional countries are meeting on Zimbabwe's political crisis.

South Africa's president is joining the Angola-hosted summit of the Southern African Development Community, along with the leaders of Zambia and Tanzania.

A committee of the regional bloc has recommended a full summit of all 16 members to discuss Zimbabwe.

And also on Tuesday, Botswana's government posted online what it calls an open letter from President Ian Khama urging Mr Mugabe to step down.

Mr Khama has openly criticised his neighbour in the past. The letter asks the world's oldest head of state to "be sensitive to the wishes of the people of Zimbabwe and to do the honourable thing by voluntarily relinquishing power."