The protesters at 'The Time is Now Rally' set off their alarm clocks as a message to MPs.
Former Archbishop of Canterbury Lord Rowan Williams said his faith underpins his environmental concerns.
"We're told to see our material environment as part of the way God communicates with us," he said.
"As Christians we believe in a God who actually works in flesh and blood - in the life of Jesus. So all of those things for me make it a real theological imperative."
The mass protest has been organised by the Climate Coalition and Greener UK, whose members include Catholic international development charity CAFOD.
CAFOD Director Christine Allen told attendees of an interfaith event at Church House on Wednesday: "This is our opportunity as people of faith to come together before we meet later with our MPs at this amazing gathering.
"We come as people united in faith, united in our relationships and united with our world.
"Humanity still has the ability to work together for our common home - today we will join with 16,000 people for the good of our common family and the world.
"To meet with our MP helps to bring a new hope and a new vision. We stand together."
MPs have approved a motion to declare an environment and climate emergency and the lobby comes on the day the Lords debate a target to cut greenhouse gas emissions to net zero by 2050, which should pass into law this week.
Campaigners will be pushing their MPs to back the policies needed to turn the new net zero target into a reality.
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