Nine activists of the Black Lives Matter campaign group have been arrested after chaining themselves together on the runway of London's City Airport, disrupting all flights in and out of the busy travel hub for six hours.

The process of removing the protesters began at 9.30am (08:30 GMT) on Tuesday, but it was not until around 11.30am local time that the runway was cleared of protesters, who had "erected a tripod and ... locked themselves together", the Metropolitan Police said.

Tweeting that the runway had reopened, London City Airport apologised for the disruption and advised travellers to check with their airline for flight information.

More than 120 flights were cancelled, delayed or diverted as a result of the protest, which started at about 5.30am, according to police.

Runway is now open, we apologise for inconvenience caused this morning. Passengers are advised to check with airline for latest flight info. — London City Airport (@LondonCityAir) September 6, 2016

The Twitter page of Black Lives Matter UK claimed responsibility for the protest, saying it was to highlight the "UK's environmental impact on black people".

The protesters at the airport had put up two large posters with the slogans "Black Lives Matter" and "Climate Crisis is a Racist Crisis".

While the movement started in the United States as a reaction to the fatal shooting of black people by police, the British arm of the group said that it wanted to highlight Britain's environmental impact on the lives of black people, locally and globally.

"Black people are the first to die, not the first to fly, in this racist climate crisis," the group said in a statement.

"When black people in Britain are 28 percent more likely to be exposed to air pollution than their white counterparts, we know that environmental inequality is a racist crisis," the group said, adding that City Airport was designed for the wealthy while those who lived near the site struggled on low salaries.

Flights diverted

Flights due to land at the airport were diverted to Southend and Gatwick airports near London. Pictures from the airport showed the protesters lying down on the tarmac beneath a bamboo tripod.

The UK is the biggest per-capita contributor to temperature change & among the least vulnerable to its affects. pic.twitter.com/Bb5SmKWyt6 — #BlackLivesMatterUK (@ukblm) September 6, 2016

London's City Airport is located in the docklands east of the city centre, and close to Canary Wharf, one of the two centres of London's finance industry.

The airport offers short-haul flights to locations in Britain and mainland Europe, and served more than four million passengers last year, many of them travelling on business.

Its sole runway is surrounded by water, but it was reported that the protesters were able to swim from land surrounding the airport, raising questions about security.

Activists from Britain's Black Lives Matter blocked the main road into London Heathrow, Europe's busiest airport, last month as part of a nationwide day of protests against racism.