People showered the parade route with confetti in a joyous celebration that sharply contrasted with the flame's chaotic last stops in Europe and the United States. No violence was reported from small groups of fenced-off demonstrators protesting China's human rights record, who exchanged jeers with pro-China demonstrators.
Activists had promised "entertaining surprises," but the biggest threat to the torch's tour seemed to be blustery winds that caused the propane-powered flame to flicker. Protesters threw at least three water balloons at the torch as it passed the presidential palace, but guards easily batted them away.
"This is beautiful, a marvelous spectacle," said Marcelo Tejera, 26.
Banks, government offices and businesses took an impromptu half-day holiday for the only Latin American stop on the Olympic flame's five-continent journey from ancient Olympia.
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.
Rowers sped the flame down a muddy River Plate canal, their long oars flashing beneath gathering storm clouds, before runners on land jogged past the pink presidential palace.
Heavy security accompanied the torch. About 1,300 federal police, 1,500 -naval police and 3,000 traffic police and volunteers guarded the 8½-mile (14-kilometer) relay route, and at least four security layers swaddled the torchbearers.
A tight group of Chinese guards wearing Argentina's blue-and-white surrounded the runners, with riot police driving alongside. Farther out, a line of burly men in blue-and-black track suits linked hands in a moving cordon, and plainclothes federal police patrolled beyond that.
Liu Qi, head of the Beijing organizing committee, told senior International Olympics Committee officials in Beijing on Friday that additional steps had been taken to protect the flame, IOC spokeswoman Giselle Davies said "we're very confident and comfortable with that."
About 500 China supporters in red windbreakers handed out by organizers waved banners and denounced the political protests that disrupted the flame's last stops in London, Paris and San Francisco.
"We are here to celebrate the Olympics," said Shao Long Chen, a 19-year-old Chinese immigrant. "It's a great source of pride for us that the Olympics are being held in Beijing and that the torch is passing through Buenos Aires."
As for the protesters nearby, he said: "They're using sports to deliver a political message, and that's not right."
Protesters say China doesn't deserve to host the Olympics 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 against Beijing as 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 ceremony began with a tango performance along a canal. The torch passed by the pink Government House and the iconic Obelisk, and was ending at an equestrian club with Argentine tennis great Gabriela Sabatini as the last runner.
Following the relay, the torch is to be flown to Tanzania.