Hundreds of pro-life protesters listened to the leading Catholic and pro-life peer who condemned the "willful killing of the smallest and most helpless member of the human family in the very place she should be safest - her mother's womb".
He said: "What we mark today, is nothing less than the greatest shame of our nation. The fact that we neuter the protections in our laws for our tiniest countrymen and countrywomen is a horror and disgrace on a massive scale.
"We are here to signify that we are not going away, that we are not going to remain quiet while injustice occurs and we are not going to stop working and educating and championing and fighting until we live in a society where the humanity, dignity and the rights of every member of our nation are together recognised."
On 27th October 1967, the Speaker of the House of Commons declared that Royal Assent from Her Majesty the Queen had been given to David Steel's abortion Bill. The Abortion Act 1967 came into effect six months later on 27th April 1968.
Lord Alton detailed the effect of the Act.
He explained: "Since then, 8,894,355 abortions have been carried out, and at least that number of unborn children have lost their lives. I say 'at least', because of course some single abortions will have been carried out on twins, or triplets, or other multiple pregnancies.
"That is a monstrous figure."
On Friday, public education project, Abort67, unveiled a pro-life display called 'Moving Truth' with images of aborted foetuses that circled Parliament Square and travelled up and down Whitehall.
The group, which works closely alongside campaign group Christian Concern, said its intention was not to upset people, but to "save lives".
Lord Alton echoed this point and called on people to "succeed in establishing a lasting justice and a true peace".
He added: "I don't want you to be standing here in another 50 years lamenting another 8 million lives lost. The torch of compassion and human progress is now passing to you.
"We must work to end this killing in your lifetime, and we can only do that if every person here takes seriously the role they must play in making sure that this happens."