Stanford University students have begun an indefinite sit-in to protest against the institution’s investments in fossil fuels.

According to protest organisers, more than 100 students attempted to enter the main administration building and office of the president on Monday afternoon. The students have been locked out but say they have encircled the building and will camp out there until Stanford acts on their demand to “completely divest” its financial holdings from the fossil fuel industry.

They are demanding action before UN climate negotiations in Paris at the end of November.

In May 2014 Stanford agreed it would not invest any more of its $22.2bn of endowed funds in coal mining companies, and would divest any current holdings. The pledge followed a campaign by student-led organisation Fossil Free Stanford, the group behind the latest protest, which now says that after three years of campaigning and negotiating the administration is not acting quickly enough.

Around 100 students gathered outside the university’s main administration building demanding that divestment plans be accelerated. Photograph: Jae-Young Son

Repeated calls for the university to move beyond coal to full fossil fuel divestment, which also covers oil and gas, have not been heeded. “We refuse to let Stanford stand idly by and continue to remain invested in the companies perpetuating the climate crisis,” said Michael Peñuelas, a Fossil Free Stanford organiser and master’s student.

Emma Walker-Silverman, a Stanford undergraduate taking part in the protest, said students were planning to remain put – even if that meant sleeping outside – until a commitment was given.

“We will be here until we see meaningful action from our administration,” she said. “Dramatic climate change isn’t an issue that you can just hope goes away … We want to send a strong message that this model of business is no longer acceptable.”

Stanford University spokeswoman Lisa Lapin said the university respected the right of its students to demonstrate and the protest had so far been peaceful. The students’ request for fossil fuel disinvestment was “under review” she said, adding there was a “campus process” for it and the group had met with Stanford president John Hennessy on 11 November where he told students the issue was still “in process”. “We are a leader amongst universities in terms of all of our actions related to reducing our impact on the climate,” she added.

John Hennessy, who was having a haircut at the university’s salon when the protests began, told the student news site The Fountain Hopper that he had no desire to meet them today and that students should make an appointment to discuss their concerns directly.

Stanford president John Hennessy, who was having a haircut as protests began, and said he had no desire to meet students today Photograph: The fountain Hopper

“[Divestment] is a process that APRL [Advisory Panel on Investment Responsibility and Licensing] is going to undertake,” he said. “It’s a trustee decision in the end and they are going to start that process … the discussion [about divesting from specific companies] has only just started and is going to take them some time.”

As a private institution Stanford does not disclose specific investments in its portfolio nor their individual value. It is understood that Hennessy was not in the building at the time the students tried to enter.

Stanford is a private institution with around $22.2bn of endowment funds that include fossil fuels. Photograph: Jae-Young Son

Fossil Free Stanford is part of a national divestment campaign which has targeted universities to shed their shares in fossil fuel companies. According to Go Fossil Free 13 US universities or their philanthropic foundations have either partially or fully committed to divesting of fossil fuel investments, but Stanford would be the first major private university to divest entirely.

Ten UK universities are in the process of moving their money out of fossil fuels ahead of crunch UN climate change talks in Paris later this month.