Protesters wearing the colours of both Harvard and Yale have staged a sit-in at midfield of Yale Bowl during halftime of the 136th edition of the annual football rivalry known as The Game.

Key points: Protesters stormed the field at halftime of the Yale-Harvard football game, delaying the match by an hour

Protesters stormed the field at halftime of the Yale-Harvard football game, delaying the match by an hour Demonstrators held signs protesting inactivity to address climate change

Demonstrators held signs protesting inactivity to address climate change The Yale vs Harvard football match is known as "The Game" and was first played in 1875

Most walked off after about an hour with a police escort, but 20-30 protesters who remained were arrested.

A few dozen protesters initially trickled onto the field as the Yale band finished performing its halftime routine, some holding a banner asking the schools' presidents to divest from the fossil fuel industry. Other signs referred to Puerto Rican debt and the treatment of the Uyghurs.

Yale officials said in a statement handed to reporters in the press box during the fourth quarter that the school "stands firmly for the right to free expression".

"It is regrettable," a statement attributed to the Ivy League said, "that the orchestrated protest came during a time when fellow students were participating in a collegiate career-defining contest and an annual tradition when thousands gather from around the world to enjoy and celebrate the storied traditions of both football programs and universities."

Loading

Rachel Sadoff, a junior at Harvard, said about 150 students from the two universities planned to participate and about 100 more who had been sitting in the stands joined in.

"Our goal was to spread the word," Ms Sadoff said.

"If more people speak up, our colleges will have to listen."

Largely of college age but with a few older protesters mixed in, the group chanted: "Hey Hey! Ho Ho! Fossil fuels have got to go!"

One banner read "This is an emergency", with one other stating "Nobody wins".

Mostly, protesters sat or milled around near midfield, with some taking selfies; a vape pen and a crushed can of beer were left behind.

Police in yellow vests lined up alongside the sit-in but did not intervene. When the 15-minute halftime expired and the protest continued, hundreds more fans streamed onto the field to join in. Fans remaining in the stands began to boo, but only briefly.

'Ok, boomer'

The public address announcer implored the group to leave, repeating: "As a courtesy to both teams, the game must resume."

The protest interrupted the 136th edition of the Yale-Harvard rivalry football match. ( AP: Jimmy Golen )

Protesters responded by chanting, "OK, boomer."

Loading

Players tried to remain warm on the sideline in temperatures below 10 degrees Celsius, but then returned to their locker rooms. Harvard coach Tim Murphy was given an update from the game officials and public safety officers as the protest continued.

Yale police chief Ronnell Higgins spoke to the protesters over a megaphone, trying to convince them that they had made their point, but it would be lost if the situation escalated.

After about an hour, police formed a line and moved forward, from the Yale sideline toward the Harvard sideline. A protest leader encouraged all "internationals" to leave. An agreement was reached to escort the remainders off, with one police officer to every two protesters.

Those who did not leave then were informed by Chief Higgins that they would be arrested.

Messages left with Yale and New Haven police were not immediately returned.

In the match, Yale rallied from a 17-point fourth-quarter deficit to win 50-43 in double overtime in darkness in the lightless, century-old Yale Bowl to clinch the Ivy League championship and conclude one of the strangest editions in the rivalry's 144-year history.

Supporters from the stands joined the protest in the centre of the historic Yale Bowl stadium. ( New Haven Register via AP: Arnold Gold )

AP