Greta Thunberg is mobbed as she arrives in Glasgow.

Greta Thunberg’s arrival in Glasgow on Saturday for the United Nations climate conference quickly unfolded into a scene of chaos as the activist was mobbed by dozens of people.

Ms. Thunberg, 18, was not scheduled to speak at the 12-day summit, which started on Sunday, but she arrived anyway by train into Glasgow. She was among the many activists who have descended on the COP26 summit to demand that world leaders take measures to slow down catastrophic climate change.

Ms. Thunberg, whose solo climate strikes in 2018 helped fuel a global youth climate movement, was quickly surrounded by a raucous crowd after stepping out of a gate at Glasgow Central Station, according to videos of the scene posted on social media.

She did not appear to speak to anyone who had surrounded her after they greeted her with a mix of cheers and yells, according to the videos. She kept her head down and followed the police officers who were escorting her through the crowd, which appeared to include photographers, young people and one heated man who admonished the people gathered.

“Have some compassion. You’re not entitled to her,” he told a photographer in one of the videos.

Another man said: “Give her some space. This is not right.”

Ms. Thunberg told the BBC in an interview last week that she had not been “officially” invited to speak at the summit. She added that she thought the summit organizers had not invited a lot of young speakers because they “might be scared that if they invite too many ‘radical’ young people then that might make them look bad,” she said, using air quotations.

It was unclear how long Ms. Thunberg planned to stay in Glasgow. She was scheduled to join a Fridays for Future climate strike in the city on Friday.

In the Glasgow train station, after leaving most of the crowd behind her, she descended an escalator and raised her fist in the air. She then gave a thumbs up to those who had gathered.

It was hard to read Ms. Thunberg under her mask, but it appears that she appreciated the reception in the train station. She said on Twitter on Saturday, “Finally in Glasgow for the #COP26! And thank you for the very warm welcome.”