Revelations of the sexual abuse of children by priests at Catholic institutions have swept across Europe, including in the pope's native Germany.
Benedict has been criticized over a case dating to his tenure as archbishop of Munich, and his actions when head of the Vatican office responsible for disciplining priests.
"The buck stops with him and he should resign," human rights campaigner Peter Tatchell said at the London rally. "The pope has played a direct personal role in covering up sex abuse."
However, Benedict won support from Archbishop of Westminster Vincent Nichols who insisted the pope wouldn't – and shouldn't – quit.
"The pope will not resign, frankly there is no strong reason for him to do so," he told BBC television. "In fact, it is the other way around. He is the one above all else in Rome that has tackled this thing head on."