Hundreds of protesters, advocating a variety of causes, braved torrential rain as they walked through streets.
Spirits were high and there were initially no signs of the disorder that some in Belfast had feared.
Many shops front have been boarded up in case the protest turned ugly.
Hundreds of public order police officers flanked the route with scores of fortified Land Rover-type vehicles also parked up.
Police helicopters circled overhead.
Environmentalists, trade unionists and other civil society activists are parading through city ahead of rally at Belfast City Hall later.
The G8 leaders are arriving at the Lough Erne golf resort in Fermanagh for the two-day meeting starting on Monday.
A separate concert for the IF anti-food poverty campaign, spearheaded by charities working in the developing world, will be held in the city’s Botanic Gardens this afternoon with acts including indie rockers Two Door Cinema Club.
The concert has been sold out, with around 8,000 people due to attend, organisers said.
Police estimated that around 1,500 people took part in the march through Belfast.
As participants gathered at City Hall for speeches, a separate demonstration was being staged by Northern Ireland-based loyalists unhappy at last year’s decision by Belfast City Council to limit the number of days the Union flag flies over City Hall.
Similar flag protests have been staged outside City Hall on Saturdays since the controversy flared.
Around 100 loyalists congregated near the gates of the landmark building with around the same number of police officers forming a human barrier to ensure they were kept apart from the anti-G8 rally.