Efforts to drop a banner protesting Oxbridge investments in the fossil fuel industries during this year’s Oxford vs Cambridge Boat Race in London were prevented by police.

Around 40 climate activists from Oxford and Cambridge had planned to stage a banner drop at the annual race, over Hammersmith Bridge, with the 20-metre cloth reading “Oxbridge Come Clean”.

A number of activists were stopped and searched, and the bag containing the banner was searched around 3pm, as the men’s boats passed underneath the bridge. The banner was later returned to the activists.

The banner-drop was organised by Cambridge Zero Carbon Society and Oxford Climate Justice Campaign, both of which are student-led activist groups campaigning for divestment in their respective universities.

The protest against the universities’ investments in the fossil fuel industries was intended to take place as hundreds of thousands of spectators lined the River Thames, and a predicted audience of millions tuned in to the BBC to watch the annual race.

A statement from Zero Carbon said that the “aggressive manner in which police acted at today’s peaceful, legal protest is unsettling, if not unsurprising.”

“Having seen their democratic will suppressed within the University environment, students now find their freedom of expression impeded even in a supposedly “free” public space.”

They added: “It is a testament to the power of the politics of climate justice, and of our growing movement, that the authorities are willing to go to such lengths to silence it. This merely inspires us to further struggle.”

During last year’s men’s race, climate activists dropped an ‘Oxbridge divest’ banner over Hammersmith Bridge and released orange smoke grenades. Last year’s protest did not disrupt the race itself, although the flares were captured by BBC helicopter footage, and a Varsity reporter was told that this year’s was intended to be similarly peaceful.

Protestors had been on the bridge since 9.30am, and claimed that police presence had been visible since the morning.

A Varsity journalist was told by a policeman present in the morning that they were looking out for action from student activists.

With an estimated 18 police officers in attendance, concentrated where the protestors were standing and removing individuals from the crowd throughout the day for questioning, the groups said they “felt that this was a concerted effort to silence our protests, and this suspicion was later confirmed when we were on the bridge.” Protestors who were spectating were also searched, with the groups’ statement adding that they were asked for personal information.

Closer to the time of the proposed protest, one justification provided for the searches were the zip ties being carried by some of the activists in attendance. While the bag containing the banner was searched, individual activists were separated and surrounded by police.

The Metropolitan Police Service has been contacted for comment.

The banner-drop was the latest in a series of ongoing efforts to put pressure on Oxbridge to divest. Previous action at Cambridge in the past year has included an occupation of Greenwich House, the University’s central administrative building, a disruption of the Shell annual lecture, and banner drops across 11 colleges to protest college fossil fuel investments.

Despite increasing pressure from staff and students for full divestment, Oxford pledged partial divestment from coal and tar sands in 2015.

In both universities, college-level change has taken place. This February, Cambridge’s Clare Hall passed a policy favouring divestment, following Downing, Selwyn, and Queens’. Last year, both St Hilda’s and Oriel in Oxford adopted divestment policies.

Last year, Cambridge’s University Council decided to reject full divestment, based on the recommendations of the Divestment Working Group. In January, The Guardian revealed that two members of the Working Group had not disclosed major conflicts of interest.

This story has been updated as more information became available, at 17.44, 18.56 and 22.55.