There was no sign of violence on the "Migrants' March", called to demand better rights and freedoms for immigrants and refugees, as demonstrators chanted and danced through the streets of the port.
This march skirted around the high-security red-zone of the city but more radical groups have plans to enter the zone by force.
In total about 100,000 protesters are expected to gather in the city, for what some activists have described as the most ambitious event in two years of disrupting summits.
Police presence
Thousands of protesters, many wearing protective body padding, started arriving overnight.
The Genoa Social Forum (GSF), an umbrella group for around 700 protest groups, has been schooling them in self-defence ahead of expected clashes with the security forces.
Fearing the kind of violence that marred last month's European Union meeting in Gothenburg, the authorities have turned Genoa into a virtual ghost town.
Click here to see a map of the city
Around 20,000 police and paramilitary forces, equipped with automatic rifles, helmets and riot shields will be on hand to guard the talks.
"Today I don't think there will be problems, it will be a peaceful demonstration. It's just a shame to see the city entirely closed down," said one participant in the Migrants' March.
"The situation of migrants in Italy [is] the same as the people of Genoa during the G8... the people in the red zone in Genoa are living the experience of migrants in everyday life," said Alessandro, a university professor on the march.
Hundreds of police officers watched the march take place.
"We will try to retake every metre of the red zone," said Luca Casarini, leader of the radical Tute Bianche (White Overalls) group whose activists are staging their final preparations for Friday's offensive.
The palace where the summit will take place is protected by a four-metre-high (13-foot) steel fence to keep protesters at bay but the Tute Bianche are determined to either go through them or over them.
Shops, bars, businesses and restaurants will remain closed behind steel shutters until next Monday.
Condemnation
Protesters planning to use violence have been condemned by UK Prime Minister Tony Blair and US President George Bush.
"Small groups of anarchists and hooligans have been using legitimate protest as a vehicle for causing chaos and mayhem," said a spokesman for Mr Blair.
But European Commission President Romano Prodi has questioned whether such summits should even go ahead.
He said the summits had grown "extravagant and excessive" and that it was time to "return to an organisation which puts the priority on people."
Poverty, disease and poor nations' debt will feature high on the agenda of the two-day summit.
Foreign ministers meeting in Rome have been focusing on a range of issues, including the heightened tensions between Israel and the Palestinians, and the ethnic crisis in Macedonia.
They are also discussing controversial American plans for a new missile defence system and have said issues of particular concern to Africa, such as Aids and debt reduction, will be at the centre of talks.
Radicals are gearing up for what are expected to be violent protests on Friday to coincide with the beginning of the summit.
President Bush said anyone who demonstrates against the G8 was no friend of the poor
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