Pax Christi's British chairman Stuart Hemsley told the BBC News website he read out the names of 29 British soldiers with children, who had been killed in Iraq.
The group also picked out the names of 50 Iraqi children aged five and under.
"We had no problems from the police whatsoever, they just stood there looking stony-faced. It was as if we weren't there.
"I am not disappointed I have not been arrested but I wonder if this will now set a precedent."
He said the group of 15 wanted to pray and worship at the seat of power in the hope they would continue to raise awareness of the situation in Iraq.
Good on Pax Christi!