Morgan Tsvangirai, leader of the Movement for Democratic Change (MDC), made his appeal as he addressed hundreds of delegates at the annual conference expected to address deep economic and political problems.
"History has shown that dictators thrive in a climate of fear," Tsvangirai said on Saturday. "Let us continue to fight fear."
The conference is taking place against a backdrop of unprecedented social and economic crises, and an ever-deepening rift between supporters of the MDC and President Robert Mugabe's party.
The MDC blames Mugabe's government for the various crises, including inflation of 620%, 70% unemployment and massive food shortages threatening half the country's 11.6 million people.
Tsvangirai told his supporters that Mugabe's Zimbabwe African National Union - Patriotic Front (ZANU-PF) was "now totally incapable of handling the crisis it created".
President Mugabe is blamed for
a range of ills facing Zimbabwe
The opposition stormed onto the political stage after 2000 parliamentary elections, taking nearly half the contested seats and posing the biggest threat to Mugabe's hold on power. Since then inter-party relations have worsened.
A top party official later told delegates that there had been no progress in organising inter-party talks, a solution South Africa has been pressing for.
Tsvangirai said more pressure should be put on Mugabe's government "to force it to come to the negotiating table".
Mbeki jets in
This week South African President Thabo Mbeki jetted into Harare and held talks with Mugabe and Tsvangirai to gauge the progress towards inter-party dialogue.
Mbeki (L) and Mugabe met for
talks on Zimbabwe's troubles
"We urge President Mbeki to continue with his efforts. We are willing to work with him in this regard," said Tsvangirai.
He said he believed Mbeki had been in the country to try to "clear obstacles" in the way of the talks. Mugabe insists he must be recognised as legitimate president before talks take place, which the MDC refuses to do.
The MDC conference comes two weeks after Mugabe pulled his country out of the 54-member Commonwealth grouping of former British colonies after it prolonged his country's suspension from the group.
Zimbabwe was suspended from the Commonwealth after some international observers said last year's presidential election was marred by vote-rigging, violence and intimidation.