Between 15 and 20 people, many of them aged under 17, unfurled a banner on the pavement outside the transport hub reading “are we the last generation” on Friday morning.
The protest is part of wider demonstrations around London organised by the campaign group Extinction Rebellion (XR), which has blocked routes around Marble Arch, Oxford Circus, Parliament Square and Waterloo Bridge since Monday.
Protesters stood by the tunnel that leads to Terminals 2 and 3 at the airport, but all roads around the roundabout remained open.
Extinction Rebellion said police had warned the youngsters at Heathrow that they could be arrested.
Home Secretary Sajid Javid has urged police to use the “full force of the law” to deal with illegal XR demonstrations as they enter a fifth day.
More than 500 protesters had been arrested by Thursday night as activists continued to ignore orders to leave roadblocks at Waterloo Bridge, Oxford Circus and Parliament Square.
Organisers said action would be escalated to include Britain’s busiest airport on Good Friday, with around 500,000 people expected to fly out for Easter breaks over the bank holiday weekend.
The airport said it was “working with the authorities”, while Metropolitan Police Assistant Commissioner Nick Ephgrave said: “Protesters can expect a robust police response. We are determined to keep the airport operating.”
Scotland Yard has warned protesters the force had “strong plans” in place with a significant number of officers ready to respond.
Ken Marsh, chairman of the Metropolitan Police Federation, told BBC Breakfast on Friday that protests had been “very, very difficult” for the force because it was an “alien” situation for most of them.
But he said that with more than 1,000 officers being deployed the streets will begin to be cleared.
He added: “This is very, very difficult for us because my colleagues have never come across the situation that they are faced with at the moment.
“They are dealing with very, very passive people, probably quite nice people, who don’t want confrontation whatsoever with the police or anyone else but are breaking the law.
“We are having to adjust to that, we are having to deal with the circumstances that are put in front of my colleagues, but be very robust so we can start clearing the streets and you will see that starting to happen today.”
Mr Marsh said the protests will cost millions of pounds and local communities will suffer.
He added: “This is very, very frustrating for us, this is going to cost millions.
“The sufferers will be the communities in the local boroughs where officers are being taken from the community areas and the funding and money has to be found to deal with what we are doing.
“But the knock-on effect will be that at some stage colleagues need to have their time off, their breaks, and it needs to be paid for. We as Londoners will all suffer from what has taken place.”
Some officers have been working 12-hour shifts, while rest days and leave have been cancelled.
The Met said the protests are putting a strain on policing in the capital with officers diverted away from “core local duties”.
After a briefing from Met Commissioner Cressida Dick, the Home Secretary said: “I totally condemn any protesters who are stepping outside the boundaries of the law.
“They have no right to cause misery for the millions of people who are trying to lead their daily lives. Unlawful behaviour will not be tolerated.
“I expect the police to take a firm stance and use the full force of the law. They have my full backing in doing so.”
Mass arrests for obstructing the highway and breaching the Section 14 order that prohibits protests apart from at Marble Arch do not appear to have deterred protesters and 10 people have been charged with those offences.
A further three people who were charged by British Transport Police appeared in court on Thursday morning.
Cathy Eastburn, 51, Mark Ovland, 35, and Luke Watson, 29, were remanded in custody over their alleged involvement in obstructing trains at Canary Wharf station on Wednesday morning.
Organisers said they expected more people to join the protests and warned they would continue until their demands are met.
The group wants the Government to declare a climate emergency and take action to reduce greenhouse gas emissions to net zero by 2025.
Actress Dame Emma Thompson joined the protests in London after flying from Los Angeles.
In a video shared by the campaign group on Thursday, the Hollywood star urged viewers to “come and join” the demonstrations.
A representative of Dame Emma said she needed to take the 5,400-mile flight home to London after working in LA.