LONDON — Demonstrators grabbed at the Olympic torch, blocked its path and tried to snuff out its flame today in raucous protests of China's human rights record that forced a string of last-second changes to a chaotic relay through London.

The biggest protests since last month's torch-lighting in Greece tarnished China's hope for a harmonious prelude to a Summer Games celebrating its rise as a global power. Instead, the flame's 85,000-mile journey from Greece to Beijing has become a stage for activists decrying China's recent crackdown on Tibetans and support for Sudan despite civilian deaths in Darfur.

Demonstrators attempted to board the bus trailing the torch shortly after British five-time gold medal rower Steve Redgrave started the relay at Wembley Stadium.

Less than an hour later, a protester slipped through a tight police cordon and gripped the torch before he was thrown to the ground and arrested.

"Before I knew what was happening this guy had lurched toward me and was grabbing the torch out of my hand and I was determinedly clinging on," former children's television host Konnie Huq told British Broadcasting Corp. television.

"I do feel for the cause," she said. "I think that China have got a despicable human rights record."

Another demonstrator tried to snuff the flame with a spray of white powder from a fire extinguisher, police said. Still others threw themselves in the torch's path. They were tackled or dragged off by police. Authorities said 37 people were arrested.

Chinese state TV showed scenes from the torch relay, but did not air any footage of the demonstrations. Some of the protests and scuffles could be seen on CNN and BBC broadcasts in China, but parts of those were blacked out by Chinese censors.

Beijing Olympics organizers called protests an act of "sabotage."

"Some protesters tried to sabotage the torch relay, by trying to grab the torch or extinguish it, stirring clashes with British police," an unidentified spokesman from the Beijing Olympics organizing committee torch relay center was quoted as saying by the official Xinhua News Agency. "The act will surely arouse the resentment of the peace-loving people, and is bound to fail."

Sun Weide, a spokesman for the Beijing Olympic organizing committee, said he could not immediately comment on the London protests.

London's Metropolitan police said some 2,000 officers, on foot, motorcycles, bikes, and on horseback tried to keep the procession under control.

One group of Tibetan protesters was corralled in metal barricades across from Bloomsbury Square.

"It feels like we are restrained like a sheep in a barn," said Passang Dolne, 27, a Tibetan national who works as a nurse in London. "It really hurts."

Chinese nationals about 100 yards away were allowed to move freely as they waved Chinese flags distributed by the Chinese Embassy and the Bank of China.

"We don't like the Tibet people who use this time against the Chinese. It's not a proper venue," said Ting Yan, 27.

The demonstration swelled near where Chinese Ambassador Fu Ying was expected to carry the torch. Frantic organizers shuffled the order of participants and Fu unexpectedly appeared in the heart of Chinatown, jogging unhindered with the torch before handing it to the next runner.

"Maybe on TV screens there might be some chaotic spin," Chinese embassy spokesman Liu Weimin told the BBC from the relay convoy. "I saw more smiling faces, waving hands, and thumbs-up welcoming the Olympic relay,"

But there were ugly scenes between Trafalgar Square and Big Ben, where a dozen protesters charged the torch.

"Everyone was running at (me). It was a bit weird," said Scott Earley Jr., 17, the torchbearer at the time.

About 100 demonstrators managed to briefly impede the flame's progress by surrounding it near St. Paul's Cathedral, forcing police to put the flame on a bus before continuing.

The torch was closely followed in east London by dozens of demonstrators shouting "Shame on China!" Police stopped to form a protective phalanx three or four officers deep every time the torch was handed to a new runner.

British Prime Minister Gordon Brown briefly greeted the flame when it arrived outside his Downing Street residence.

Brown never handled the torch, but watched as Olympic gold medalist Denise Lewis handed it to Paralympic powerlifting hopeful Ali Jawad. Pro-Tibet demonstrators and police clashed yards away, near Britain's Parliament buildings.

The torch made it unscathed to the O2 Arena in Greenwich after more than seven fraught hours that belied the London event's theme: "Journey of Harmony."

"There was definitely a bit of an edge," British tennis player Tim Henman, one of the torchbearers, told The Associated Press.

Olympics Minister Tessa Jowell later said Britain was celebrating the Olympics, not China's human rights record.

"The welcome of the Olympic torch to London is not the same as condoning the human rights regime in China or condoning the treatment of Tibet," she told the BBC.

There had been scattered protests before the torch reached London, and more are expected as the flame moves on to Paris, San Francisco and New Delhi.

"They've called the torch relay a journey of harmony, but on the ground in Tibet they are shooting and killing peaceful Tibetan protesters," said Matt Whitticase, spokesman for the London-based Free Tibet Campaign. "We want to use the momentum gathered over this weekend to really press our case that the torch should not be allowed to be paraded triumphantly by China."

French torchbearers will be encircled Monday by several hundred officers, some in riot police vehicles and on motorcycles, others on skates and on foot. Three boats were also to patrol the Seine River, and a helicopter was to fly over Paris, police said.

The head of Reporters Without Borders, arrested in Greece last month for protesting during the flame-lighting ceremony there, said the group had altered its initial plans because of the heavy police turnout. Without giving away details, Robert Menard promised protests would nonetheless be "spectacular."

Two-time French judo gold medalist David Douillet said torch carriers will wear badges reading 'For a better world' — which French athletes also hope will be permitted to wear in Beijing. Paris City Hall is to be draped with a banner supporting human rights.

French President Nicolas Sarkozy has suggested he might boycott the Olympic opening ceremony depending on how the situation evolves in Tibet.

Gayle Donsky, a spokeswoman for the San Francisco Bay Area Darfur Coalition, said her group did not intend to disrupt the torch relay and was not calling for a boycott of the Beijing Olympics.

Still, she said, activists see Wednesday's upcoming relay along San Francisco's waterfront as an unprecedented chance to draw attention to China's alleged complicity in the violence in Sudan.

Associated Press Writers Regan McTarsney in London, Marcus Wohlsen in San Francisco and Angela Doland in Paris contributed to this report.