Campaigners have accused the police of taking an “increasingly authoritarian” attitude towards peaceful protest as it emerged that the cost of policing an international arms fair in London more than doubled to £2.4m.

Data obtained under freedom of information laws also showed that the Metropolitan police deployed twice as many officers – 5,609 – over a 13-day period covering the DSEI fair in September as during the event’s previous staging, in 2017. More than 120 protesters were arrested in the run-up to and during this year’s convention.

Siana Bangura, of the Campaign Against Arms Trade, accused the police of mounting an “inappropriate and heavy-handed” operation. “This is part of a bigger picture, with the police in the UK taking an increasingly authoritarian approach to protest,” she said.

The policing bill for the fair in 2017 was £978,000.

DSEI is Europe’s largest arms fair, attracting 35,000 delegates including 68 international delegations to the ExCeL centre in Docklands, east London, over four days to inspect and discuss some of the latest developments in military hardware.

Campaigners spent the days before the convention opened trying to disrupt those setting up the show, prompting police to make dozens of arrests. Some activists have subsequently been charged, although the first case collapsed last week.

Policing of the event was significantly stepped up this year, although it was not clear there was any extra security threat. In 2015 the Met deployed 2,245 officers, and in 2017 the figure was 2,810.

The Met has been embroiled in a growing controversy about its handling of protests, and in particular its response to a campaign by Extinction Rebellion activists in October.

The high court ruled that the Met’s use of a cease and desist section 14 order, which labelled all Extinction Rebellion protests in the capital as illegal, was unlawful and the force had exceeded its statutory powers in applying it.

Bangura said: “Vital services are facing cuts and austerity, with recent weeks exposing the extent to which the authorities have failed the victims of Grenfell. However, it seems like there is always money available to support the arms industry and the repression of political protest.”