Zimbabwe's Prime Minister Morgan Tsvangirai on Saturday urged support for a draft charter to pave the way for new elections to succeed the country's shaky coalition government.

"There are key progressive steps being proposed in that constitution," Tsvangirai said at a rally with about 20,000 supporters to mark the 13th anniversary of his Movement For Democratic Change (MDC) party.

The draft charter -- created by a committee of lawmakers from the two main parties -- curtails the president's powers and reduces his tenure and provides for a human rights commission and independent prosecutor.

Tsvangirai said veteran President Robert Mugabe, whose party has vowed to reject the draft if it ignores changes they propose, had indicated in private talks that the draft charter was better than the current constitution.

"If Mugabe supports it, who are you to reject it," Tsvangirai challenged those who have vowed to oppose the draft charter.

Tsvangirai formed a coalition government with Mugabe in 2009 to avoid a plunge into conflict in the aftermath of a bloody presidential run-off election in 2008.

"A coalition cannot determine the country's destiny," the prime minister said. "All the parties do is pull in opposite direction. (Mugabe's) ZANU-PF does not want it anymore. We don't want it. It's a marriage of convenience."

He urged unity in his party to ensure victory at new elections to succeed the compromise government whose tenure has been characterised by haggling over positions and counter-accusations of violence.

"Next year we will be celebrating 14 years in different circumstances with an MDC government,"he said.