Greek officials handed over the Olympic flame to organizers of the Beijing Games on Sunday amid small protests by a pro-Tibetan group.
The ceremony was held at the marble Panathenian Stadium, where the first modern Olympics were staged in 1896.
Outside the stadium, police scuffled with pro-Tibet demonstrators and prevented others from unfurling protest banners. Twenty-one demonstrators were detained - seven Indians, one Nepalese and 13 Greeks - and were all due to be released without charge, police said.
The protesters challenged a police cordon but failed to disrupt the final leg of Greece's torch relay - from the Acropolis to the stadium.
"Our aim was to call attention to human and animal rights abuses in China," detained protester Roza Minakouli, a 65-year-old environmental campaigner, told The Associated Press by telephone.
"We have no business sharing the flame with people who do not deserve it."
The protesters chanted "Save Tibet!" and unfurled a banner which read "Stop Genocide in Tibet."
Police security was tight inside the stadium and around the city following small protests by pro-Tibet and human rights groups at the March 24 flame-lighting ceremony and during the weeklong Greek leg of the torch relay.
At Sunday's ceremony, Hellenic Olympic Committee president Minos Kyriakou delivered the flame to chief Beijing organizer Liu Qi. The flame will arrive in Beijing on Monday.
"The Chinese government and its people will host a grand welcoming ceremony and officially launch the torch relay of the Beijing 2008 Olympic Games," Liu said. "The torch will for the first time ascend the summit of the world (Mount Everest), thereby testifying to the great strength of the Olympic movement in marking the progress of human civilization."
Kyriakou called for the flame to be respected during its journey.
"The Olympic flame is the timeless symbol which stirs admiration, pride and faith (in) the Olympic ideals and values," Kyriakou said. "I hope the world community welcomes the flame and honors it, showing the same feeling and necessary respect (as Greeks did)."
About 7,000 Greek and Chinese spectators inside the Panathenian Stadium watched as Greek triple jumper Hrysopigi Devetzi carried the torch into the stadium lined with Greek and Chinese flags. Greek presidential guards and actresses dressed as ancient priestesses looked on.
"It was an emotional experience for me," Devetzi said. "I hope the flame will bring light to all athletes, especially those from Greece and that everything with the games goes well."
On Saturday and Sunday, about 2,000 police were deployed around Athens.
Greek Olympic organizers have angrily condemned the protests, while two journalist groups and the small Left Coalition Party spoke out in their support.
The torch is to arrive Monday in Beijing on a purpose-fitted airliner. It then will travel through 20 countries before returning to mainland China - covering 85,100 miles.
Pro-Tibet groups are expected to stage protests along the international torch route and have already announced a demonstration in London on April 6 to coincide with the relay there.
Led by monks, the Tibet protests began peacefully on March 10 - the anniversary of a failed 1959 uprising against Chinese rule. Tibet had been effectively independent for decades before Chinese communist troops entered in 1950.
Tibetan exiles say almost 140 people have died in the violence in Tibet. Beijing puts the number at 22.