Ottawa is hosting what organizers are billing as the largest progressive event ever held in Canada: the Peoples Social Forum.

The four-day forum, which began Thursday, is concentrated around the University of Ottawa, but events associated with the forum will be happening throughout the city between Thursday and Sunday.

Organizer Mike Palecek of the Canadian Union of Postal Workers describes the Peoples Social Forum as something like a Woodstock of left-wing politics.

"These events are happening all over the place, and the social forum itself is going to feed into the pride parade on Sunday. So really, though the workshops are happening on the university campus, it's an event that's going to transform the downtown core," Palecek said.

Organizers said they've settled on 17 themes for the forum after polling participants, including immigration, gender issues and food policy.

A "Unity March" by three separate groups of protesters converging on Parliament Hill began at 2 p.m. Thursday, coming from from Jacques-Cartier Park in Gatineau, Victoria Island and the Canadian War Museum.

An estimated 2,000 people made it to the hill shortly before 4:30 p.m..

"The fact this many social movement organizations can come together in Ottawa to talk strategy in regards to this government's agenda in the last number of years, I feel pretty good (about it)," said Hassan Yussuff, president of the Canadian Labour Congress.

"I'm hopeful between now and Sunday we'll have a common understanding of what we need to do collectively to try and regain some control over this country and how we can work with each other."