Ruach City Church in Norbury told Premier it was about 11am on Tuesday morning that the caravans turned up on the land.
It's believed they got into the gates in the middle of the night.
A 24-hour notice has been served ordering the travellers to leave.
The church has been taken over before. The last time damage caused in the incident cost almost £200,000.
Congregation members have been asked to show up for a prayer meeting tonight at the church.
The Metropolitan Police told Premier there were officers at the scene but there had not been any arrests or reports of any disorder.
Last time the travellers were not moved on for three months. Because of that the church said it planned to have a vigil until the travellers leave.
"They're allowed to climb over the fence and go back in the property," said co-pastor Penny Francis.
She added: "They've been going shopping, buying supplies, I've seen them go in with a petrol tank and I've just filmed footage where one of them set fire to rubbish on the ground and said this is what we're going to do to your property".
.@BishopJFrancis on #Periscope: Ruach Norbury - MEET US!!! https://t.co/RexWQZvqfd
— fruitsoflove.... (@fruitsoflove_9) January 10, 2017
Church members were "very peaceful" at the moment, said Francis, adding that many of the travellers were being rude and appeared drunk.
"We're just trying to be peaceful and trust God that he will bring us through this. For this to happen a second time it's quite devastating", she told Premier.
The group cut the church's locks off and replaced them with their own locks. The church told Premier the police would not let them cut off the groups padlock because it would be criminal damage.
Francis added: "We will be here praying and just believing in God and putting the situation in God's hands".
She stressed that the church would not be a "bad testimony" and would not be "aggressive or fighting anyone" but the travellers must be told to "respect our land and respect God's property".
UPDATE
Ruach City Church tweeted on Wednesday morning to confirm the travellers left the land and the prayer meeting took place.
Victory! We are now in our building! Thank you to everyone who came our & supported us! Thank you for your tweets, msgs & prayers pic.twitter.com/eVBe8HTe1G
— Ruach City Church (@RuachCityChurch) January 10, 2017
