In a video shared by the campaign group on Thursday, the Hollywood star urged viewers to "come and join" the demonstrations.
She said she was unable to be at the first day of the protest because she was "away" with her husband as she turned 60.
"I absolutely wanted to be arrested on my 60th birthday but I didn't quite manage that," she said.
"I'm so proud and thrilled to be part of Extinction Rebellion."
A representative of Dame Emma said she needed to take the 5,400-mile flight home to London after working in LA.
She joined the protests on the day organisers announced plans to target Heathrow Airport at the start of the Easter bank holiday weekend.
Dame Emma said: "We will occupy until you take action because that's the only way we are going to save our planet.
"We have to be here, we have to do this. It's inconvenient for people sometimes but its much more inconvenient to leave a planet that's so completely destroyed."
More than 500 Extinction Rebellion protesters have been arrested as authorities said 1,000 officers were being deployed each day to demonstrations in London.
It comes ahead of a planned protest at Heathrow Airport on Friday, with the Home Secretary warning "the full force of the law" will be used to end their protesters' activism amid plans to disrupt flights at the UK's busiest airport.
Organiser 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.
Scotland Yard has warned protesters the force had "strong plans" in place with a significant number of officers ready to respond.
The airport said it is "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."
Mayor of London Sadiq Khan said he was "extremely concerned" about the possible disruption.
"This is extremely dangerous, illegal and is putting an unacceptable toll on our police force and our city," he said.
Some officers have been asked to work 12-hour shifts, while rest days and leave has been cancelled.
The Met said the protests are putting a strain on policing in the capital with officers diverted away from their "core local duties".
Brave reverend from @CClimateAction arrested at Oxford Street #ExtinctionRebellion just now... The vibe is still great, the message is very serious. Come down and be part of an historic moment; demanding our government act now before it's too late... #HumansofXR #rebelforlife pic.twitter.com/AhYVefdyDD— Extinction Rebellion London Ⓧ (@LdnRebellion)April 18, 2019
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 so far 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 even 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.
Large numbers of protesters moved from Oxford Circus to Parliament Square on Thursday night after organisers announced police were planning to make arrests in Westminster.
Sir David Attenborough's BBC documentary Climate Change: The Facts was projected onto Westminster Abbey.