Tsvangirai had threatened to boycott the talks, saying an election may be needed to break a deadlock in negotiations with Mugabe over control of cabinet seats in a new government.
"Monday’s platform presents us with a perfect opportunity to articulate our compelling case for equitable and sustainable power-sharing in a unity government. So we will be there…," Movement for Democratic Change (MDC) spokesman Nelson Chamisa said.
The MDC says Mugabe is trying to seize important ministries and sideline the opposition and Chamisa cited a "litany" of ZANU-PF actions he said went against the spirit of a power-sharing deal signed more than a month ago.
"You have the issue of the passport, the hate language used by state media, threats by war veterans, all those things will be presented to SADC and in the process (we will) try to make ZANU-PF see the profit of working together," he said.
Zimbabwe’s militant war veterans threatened this week to "take action" against Tsvangirai. The MDC has accused war veterans loyal to Mugabe of attacking its supporters.
Tsvangirai and Mugabe signed the power-sharing deal brokered by South Africa’s former president Thabo Mbeki on Sept 15 but it has since stalled over who should run which ministries.
South African President Kgalema Motlanthe earlier urged Tsvangirai not to boycott the summit and said only more dialogue would break an impasse over cabinet posts.
"When you seek a solution to a problem, you talk to those that you disagree with," Mothlanthe said on South Africa’s public broadcaster SABC. "You can’t make peace with your friends. You make peace with your enemies, your adversaries."
Fed up with weeks of fruitless talks, the MDC leader did not attend an emergency Southern African Development Community (SADC) summit in Swaziland this week that was to address the Zimbabwe deadlock. It was rescheduled.
Mbeki has been mediating talks but critics say he has been too soft on Mugabe and lacks leverage after he was ousted as South African president last month.
The power-sharing deal is seen as Zimbabwe’s best hope for halting a devastating economic meltdown marked by the world’s highest inflation and acute shortages of food and fuel.