Members of Christian Climate Action (CCA) sat around a banner that stated "Flying Crucifies Creation", sang hymns, read about the crucifixion of Jesus and prayed.
They aimed to join efforts in calling for government action on climate change and to highlight the impact of the aviation industry on the environment.
After the service, two members of CCA - Phil Kingston, 83, and Sheila Kingston, 80, crossed the barriers and sprayed the Heathrow Express train with chalk paint, XR symbols and climate change messages.
Police escorted both of them off the premises while a ten year old girl, Elsie Luna, stood at the barriers with a sign that read: "Thank U for giving up your freeness for my laterness."
Phil Kingston, a grandfather and former parole officer from Bristol said in a statement: "This act is for my grandchildren.
"They mean the world to me and my heart breaks for the terrible inheritance that we are leaving them. Why are we expanding Heathrow when we are in a climate emergency?"
Sheila Kingston, a grandmother and retired teacher added: "It is crucial that governments hear this important message this Easter.
"We are doing this because we are desperate. I don't know what else to do to make them listen."
Close to 700 arrests have been made in London as climate change protests continue.
On Friday a pink boat used by XR to blockade Oxford Circus was also towed away by police.
Officers are hoping to fully re-open Oxford Street on Saturday - where many shops have had to close because of the demonstrations.
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