Morgan Tsvangirai, the founding president of the opposition Movement for Democratic Change (MDC), needs to reorganise the party after it split last year in a feud over how to tackle President Mugabe's ruling Zanu-PF party.
The MDC was formed in 1999 and has been seen as the greatest threat to Mugabe's hold on power, but analysts say the split - which left the party in two hostile camps - has weakened the opposition's potency.
Nearly 14,000 delegates loyal to Tsvangirai's main MDC faction gathered in an indoor sports stadium in Harare on Saturday for a conference that could decide its future.
In a statement on the eve of the congress, Tsvangirai said the party leadership needed to find new energy to save Zimbabwe from Mugabe's "corrupt and inept dictatorship".
Tsvangirai, who accuses Mugabe of rigging three main elections in the last six years to remain in power, said his MDC faction would also debate whether it should continue to take part in elections or widen its tactics.
"The congress will take a position on whether the electoral route under the current electoral management system remains viable or whether we should widen our option to include using people power to put pressure on this regime that has reduced us to paupers and beggars in our own motherland," he said.
Mugabe, Zimbabwe's ruler since independence from Britain in 1980, has kept the opposition in check mainly by using security forces to crush any protests.
But political analysts say Zimbabweans may be mobilised by the opposition over the poor state of the economy, which many blame on Mugabe's policies. Unemployment is rising, there are food and fuel shortages and inflation rates are ruinous.