Designed as a counterweight to the World Economic Forum of business and political leaders in Davos, Switzerland, the Forum began late on Friday with calls for a new approach in the aftermath of worldwide protests against the Iraq war falling on deaf ears.
The US was singled out for some caustic criticism, though some speakers took a more diplomatic tone.
Nobel Peace laureate Shirin Ebadi accused the US of not providing justice to prisoners from the Iraq and Afghanistan wars.
Arundhati Roy, the Indian novelist and political essayist, urged activists to use the Mumbai meet to select two US companies associated with the Iraq war and launch a global campaign to shut them down.
"Last January thousands of us from across the world gathered in Brazil in Porto Alegre and we announced, we declared, we reiterated that another world is possible," Roy said.
Nobel Peace laureate Shirin Ebadi accused the US of not providing justice to prisoners from the Iraq and Afghanistan wars
"But a few miles, a few thousand miles, north of us George Bush and his men were meeting in the White House in Washington and they were thinking the same thing. They too thought another world was possible," she said.
Roy argued the Iraq war bode ill for the world at large and there was "not a single country not in the crosshairs" of US military might.
Roy is scheduled to take part in the World Social Forum and a more militant gathering known as the Mumbai Resistance due to kick off on Saturday.
The Mumbai Resistance was organised in four months by Indian leftists wary of increasing presence at the World Social Forums in Brazil of government officials and large Western non-governmental organisations.
"Their view is that globalisation can be humanized. We say globalisation must be smashed," said Mumbai Resistance organizer Darshan Pal.