The group wants a foreign policy based on "principles of peace"
High profile anti-war campaigners such as Tony Benn, Jeremy Corbyn MP and John Pilger said Tony Blair's government had "manipulated opinion" to gain support for the attack on Iraq.
Organised by the Stop the War Coalition, the meeting on Saturday adopted a peace declaration that called for the government's actions to be open to full public scrutiny.
"We need an inquiry into the whole war policy and the thousands of deaths it caused, and is still causing, not just the tragic death of Dr David Kelly alone," said the statement.
The feeling was that everything we had been saying all along had been proved right
-Stop the War campaigner
The assembly also demanded an end to the "Anglo-American occupation of Iraq" and the transfer of political power to representatives of the Iraqi people.
The British Government should now dissociate itself from further wars planned by the US "under the pretext of the 'war on terror'... and adopt instead a foreign policy based on principles of peace and social justice," they added.
Lawyer Louise Christian - who represents some of the terror suspects held at Guantanamo Bay in Cuba - and the father of one detainee were at the People's Assembly for Peace at the Friends Meeting House, in Euston Road.
The meeting also heard from former United Nations Assistant General Secretary, and humanitarian relief co-ordinator for Iraq, Hans Von Sponeck.
Mr von Sponeck resigned, after 36 years of UN service, in protest at alleged British and American violations of Security Council resolutions that made it impossible to properly distribute humanitarian aid to the Iraqi people.
He called on the international community to come to the aid of the Iraqis, but to refuse to support the "US-UK occupation".
Other speakers included trade union leaders Billy Hayes, of the Communication Workers' Union, Paul Mackney of the lecturer's union, and National Union of Journalists general secretary Jeremy Dear.
Speaking to BBC news online after the meeting, Stop the War worker Ghada Razuki said the meeting had attracted people from all parts of the country and all sections of the community.
"It was a fantastic atmosphere. The feeling was that everything we had been saying all along had been proved right. And it was clear that Mr Blair was not a popular man," she said.