Direct action group Plane Stupid, who are protesting against airport explansion and short-haul flights, allegedly entered the runway at 3.30am after cutting a hole in a fence.
A Heathrow Airport spokesman said: “A group of people have breached the airport perimeter fence and are currently staging a protest on the northern runway.
“We are working closely with the police who are dealing with the incident. The southern runway remains open.
“Our priority remains to ensure the safety of the airport community, including passengers, employees and protesters.”
They did not add whether or not the protest would cause delays to flights taking off this morning.
The protestors made sure to keep people updated with their demonstration via Twitter, where they shared several photos of themselves apparently chained together on the runway.
According to The Metro, one of the activists has apologised to any holidaymakers whose flights have been ruined.
They reportedly said: “We want to say sorry to anyone whose day we’ve ruined, and we’re not saying that everybody who wants to fly is a bad person.
“It’s those who fly frequently and unnecessarily who are driving the need for expansion, and we cannot keep ignoring the terrifying consequences of flying like there’s no tomorrow.”
The Metropolitan Police have confirmed that they are in attendance at the demonstration - and that they are working alongside Heathrow Airport staff to safely remove the protestors.
A spokesman said: “At approximately 3.45am officers were alerted to a number of protesters who had made their way airside at Heathrow Airport.
“A small number of these protesters are believed to have chained themselves to a ‘tripod’.
“Officers are in attendance.”