Pakistani pop band Junoon took centre-stage at the start of the fourth annual World Social Forum (WSF) to deafening applause from members of hundreds of grassroots organisations, who will discuss issues ranging from unfair global trade to racism and crimes against women.
"We are here to protest against war, to tell governments they need to respect human rights," Iranian Nobel Peace Prize winner Shirin Ebadi told reporters at the opening of the six-day gathering.
A more militant tone was struck by Indian novelist Arundhati Roy who urged the activists to choose two US companies to shut down.
"If we are against imperialism then we must not just support the resistance in Iraq; we must become the resistance in Iraq," Roy said to loud applause.
"So I suggest ... we pick by some means two companies - two American companies - that have profited from the destruction of Iraq, we put out a list of their offices all over the world, put up a list of every project they are handling and we shut them down," she said.
"In the great cities of Europe and America where a few years ago these things would have only been whispered now people are openly talking about new imperialism, about the 'good type' of imperialism and the need to police a new world, an unruly world."
Abdul Amir al-Rekaby, a left-wing Iraqi writer and activist whose name was added to the opening ceremony at the last minute, elicited more cheers when he spoke of mass protests for self-rule in the southern Iraqi city of Basra.
"Day after day the situation is becoming worse," said Rekaby, a dissident under Saddam Hussein's regime who heads the small Iraqi National Democratic Current.
"The resistance in Basra, Najaf and in Baghdad needs to know and feel that you are there for them," he said.
Jeremy Corbyn, an MP in Britain's ruling Labour Party who opposed the war in Iraq, tempered the anti-US tone by saying, "We are not against the American people. We stand with those who are fighting for peace."
Organisers say 1,000 Americans are among the 78,000 people who registered by the eve of the World Social Forum. Another 22,000 signed up on the last day but it was impossible to know exactly how many people were attending, forum spokesman Gautam Mody said.
The forum kicked off with an introduction by Indian dancer and social activist Mallika Sarabhai, who said she wanted to welcome allies from India's rival Pakistan.
At her cue, the band Junoon raced on stage and broke into a blend of hard rock and Sufi mystical music in front of a sea of flags of countries from Brazil to Bangladesh.
Indian communists in starched white shirts and European students in slinky black skirts danced to the beats of Junoon and later to a Zulu drum troupe from South Africa.
Groups of Indians, clad in saffron turbans, peacock feathers and swinging spears, danced vigorously while Tibetan women swayed gently in another corner of the sprawling grounds in suburban Mumbai as flags and banners from different countries fluttered in the evening breeze.
Intellectuals and students in trendy clothes sat alongside lower caste Hindus united in the belief "another world is possible", a slogan coined at the earlier WSF meeting in the Brazilian city of Porto Alegre.
The WSF, designed as a counterweight to the World Economic Forum scheduled in Switzerland later this month, has become an annual rallying point for people who believe globalisation hurts developing countries.
The first three World Social Forums were held since 2001 in the Brazilian leftist stronghold Porto Alegre, where last year the meet began a staging ground for protests against the imminent war in Iraq.
WSF opens a day after Indian Deputy Prime Minister Lal Krishna Advani vowed to turn India into a developed nation by 2020 as his Hindu nationalist party prepares for early elections.