Small groups of fenced-off demonstrators protesting China's human rights record exchanged jeers with hundreds of pro-China demonstrators, but there were no major disruptions. Three water balloons thrown at a torchbearer as he passed the presidential palace were easily batted away by guards.
The biggest threat seemed to be blustery winds that caused the propane-powered flame to flicker repeatedly.
"This is beautiful, a marvelous spectacle," said Marcelo Tejera, 26.
Mayor Mauricio Macri took the torch from Chinese organizers and opened the relay by passing it to three-time Olympic windsurfing medalist Carlos Espinola, who jogged into Buenos Aires streets flanked by Chinese bodyguards. Heavyset police from Argentina's navy huffed to keep up.
Another torchbearer climbed into a shell and rowers sped the flame down a muddy River Plate canal, their long oars flashing beneath gathering storm clouds. Back on land, runners jogged past the pink presidential palace and the iconic Obelisk.
Tennis Hall of Famer Gabriela Sabatini capped tlympics because of its human rights record, its harsh rule in Tibet and its friendly ties with Sudan. Pro-Tibet demonstrators tossed lotus flowers onto the route in what they said was a nonviolent protest of the Olympic host.
About 25 Falun Gong supporters lit a "human rights torch" and marched along the route to protest China's ban on the spiritual movement. Some traded insults with China supporters, but no violence was reported.
The flame heads next to Dar es Salaam, Tanzania, the flame's only stop in Africa.