LONDON — Demonstrators grabbed at the Olympic torch, blocked its path and tried to snuff out its flame yesterday 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.

International Olympic Committee head Jacques Rogge says he's "very concerned" about the unrest in Tibet and other international issues surrounding the Beijing Games.

"I'm very concerned with the international situation and what's happening in Tibet," Jacques Rogge said today in Beijing. He was in the Chinese capital to meet with officials from national Olympic committees.

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."