A spokesman for direct action group Plane Stupid said the demonstrators had cut through a fence and entered the runway at 3.30am.
Heathrow was selected as the site for a new runway in south east England earlier this month, causing disappointment for some who oppose airport expansion.
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.
“This would massively increase carbon emissions exactly when we need to massively reduce them, that’s why we’re here”.
‘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.
“The royal household would not speculate on any matter relating to a proposed third runway before any formal decision has been taken by those responsible for this”, the representative said.
Heathrow Airport issued the following warning on its website at 07.15am United Kingdom time this morning: “Due to protest activity at the airport, there may be delays and cancellations and you are advised to check your flight status below before travelling to the airport”. And we mean it’.
The Met Police said the protesters had chained themselves together using a tripod which they had assembled themselves.
Some of the protesters have chained themselves to fencing and have told officials they will stay there for “as long as possible”.
Prime Minister David Cameron has said Britain will make a final decision on the 23 billion pound ($36 billion) plan for a third runway at Heathrow Airport by the end of the year, setting the stage for a battle with environmentalists over the issue.