Hundreds of students have disrupted the annual Harvard-Yale football game in a climate change protest.
They invaded the field in New Haven, Connecticut, at half-time, demanding that the two elite US universities stop investing in fossil fuels.
As officials appealed for them to leave, spectators and some players also joined the protest, US media report.
About 50 people were escorted from the field by police, while others left voluntarily.
The protest began when dozens of students and alumni stormed the field, linking arms and holding signs reading Yale and Harvard United for Climate Justice, the Harvard Crimson newspaper reported.
Some chanted "Disclose, Divest, and Reinvest".
Divestment refers to the shedding of stocks, bonds or other investments as a way to tackle climate change.
The protest delayed the game by about half an hour.
In a video released by the group Divest Harvard, university football team captain Wesley Osgbury said both universities were investing in industries that are "destroying our futures".
"When it comes to the climate crisis, no-one wins," he said.
"Harvard and Yale can't claim to truly promote knowledge while at the same time supporting the companies engaged in misleading the public, smearing academics and denying the truth. That's why we are joining together with our friends at Yale to call for change."
A Yale spokeswoman said that while the university supported the right to freedom of expression it did not approve of the protesters' tactics, or the disruption of university events.
Harvard said it did not believe that divestment was the best way to tackle the climate crisis.
In a statement published by the Harvard Crimson, spokeswoman Rachael Dane said: "Universities like Harvard have a crucial role to play in tackling climate change and Harvard is fully committed to leadership in this area through research, education, community engagement, dramatically reducing its own carbon footprint, and using our campus as a test bed for piloting and proving solutions."
Some political figures took to Twitter to support the protest.
Among them was Democratic presidential candidate Bernie Sanders who told the students: "We are with you in this fight."
When people come together to stand up for justice, we win. Congratulations to the young people demanding a sustainable future for our planet. We are with you in this fight. https://t.co/l86n85UIcy— Bernie Sanders (@BernieSanders) November 23, 2019
The football game - the 136th between the two universities - resumed after the protest and was won by Yale 50-43.