At least a thousand people were left standing in the rain as the European Social Forum was formally declared open in Southwark Cathedral.
There was a potentially dangerous build-up of people outside the cathedral as several thousand arrived in the churchyard to attend the event. Frustrated visitors, including guests and Forum delegates from abroad, staged an impromptu meeting on the south side of the cathedral.
Meanwhile Spanish students, prevented by police from entering the Lancelot Link on the north side, started a chant which threatened to disturb the official speeches which had already been delayed.
Those inside the cathedral heard Ken Livingstone welcome guests from across Europe to "the largest conference in the history of Britain in over 2,000 years".
Sinn Fein leader Gerry Adams said that he wanted to see the international community cancel third world debt and promote peace, freedom, human rights, tolerance, justice and equality.
"Irish republicans are committed and determined to play our full part in working with others to achieve this.
"The European Union has an increasing involvement in all our lives, especially in Ireland. I listen to talk of a united Europe. But there cannot be a united Europe without a united Ireland and part of the forum discussion must focus on ending the partition of Ireland.
Dr Aleida Guevara, the daughter of Che Guevara, quoted Mother Teresa to the audience and claimed that socialism was still a possibility.
The ESF is a gathering of mainly young people opposed to war, racism and corporate power and in favour of global justice, workers' rights and a sustainable society. The ESF emerged from the the World Social Forum held in Brazil in 2001. Its first two ESF gatherings were in Florence and Paris.
London's accommodation for delegates is at the Millennium Dome in Greenwich.