By Aaron Ross

KINSHASA (Reuters) - Hundreds of thousands of people lined the streets of the Congolese capital Kinshasa on Wednesday to welcome home opposition leader Etienne Tshisekedi after a nearly two-year stay overseas for medical treatment.

Tshisekedi's return to delirious crowds flashing victory signs comes at a crucial moment in Democratic Republic of Congo, as a near-certain delay to a presidential election slated for November risks triggering violence in the chronically unstable central African nation.

President Joseph Kabila, in power since 2001, is required by term limits to step down this year, but opponents accuse him of delaying the Nov. 27 poll to cling to power. The government says logistical and budgetary constraints make it unrealistic to hold the election on time.

Kabila's opponents hope that Tshisekedi's return can rally people to the streets after opposition protests over the last year failed to attract large turnouts.

Some supporters carried banners with Tshisekedi's picture calling him president of the republic.

"He is the hope of all people," said Eric Ilunga, a 31-year-old businessman who awaited Tshisekedi's arrival outside Kinshasa's main airport.

The 83-year-old politician, who left Congo in August 2014 for unspecified medical treatment in Brussels, has been visibly frail in public appearances over the last two years and leaned on his son as he slowly descended the stairs of a private plane.

A girl in a white dress greeted him with a bouquet of flowers while police linked arms outside the airport to keep the crowd from rushing in. People looked on from rooftops and along the highway on the 17 km (11 mile) journey to his home.

Tshisekedi, who formed Congo's first organized opposition platform, the Union for Democracy and Social Progress (UDPS), under longtime autocrat Mobutu Sese Seko in 1982, was runner-up in Kabila's 2011 re-election, a vote observers said was marred by massive fraud.

He is scheduled to speak at an opposition rally on Sunday.

Allies had said he would lead the opposition in a national dialogue called for by Kabila expected to begin next month.

On Sunday, however, Tshisekedi said the UDPS and allied parties would not participate in a dialogue led by the African Union's designated facilitator, former Togolese prime minister Edem Kodjo, whom they accuse of bias.

Though other opposition leaders have gained prominence during Tshisekedi's time abroad, he remains by far the most popular opposition figure despite concerns over his health.

"The return of Tshisekedi represents the beginning of the departure of Kabila," said Martin Fayulu, another opposition leader.

(Editing by Nellie Peyton and Robin Pomeroy)