Police banned all traffic from the centre of Rome
Trade union leaders say they are now planning a general strike after hundreds of thousands of demonstrators brought the centre of the capital, Rome, to a standstill on Saturday.
The rally was initially targeted against Prime Minister Silvio Berlusconi's plans to make it easier for companies to dismiss employees.
But the focus was broadened into a general protest against political violence after the murder only four days earlier of government adviser Marco Biagi who had drafted the new measures.
Demonstrators brought in from all over the country on 9,000 buses, 60 trains, three ships and two planes turned the city's Circus Maximus - the site of ancient Roman chariot races - to a sea of red flags and banners.
The organisers and TV stations estimated the numbers at two million, although the police put the number lower.
They said at least 700,000 people had converged on the city centre for the noisy but peaceful rally.
Respect
The protesters bowed their heads for a minute's silence out of respect for Biagi, who was shot dead outside his house in Bologna in an attack claimed by an offshoot of the Red Brigades, an ultra-left guerrilla group active in the 1970s and 1980s.
Biagi was an adviser to the government on employment issues and co-author of controversial labour reforms that would make it easier for employers to hire and fire workers.
Workers' leaders have stressed that they want to distance the labour movement from the murder.
"We are here to fight terrorism, to support democracy and to show the government its intentions are wrong," Sergio Cofferati, leader of CGIL, Italy's biggest federation of trade unions, told the rally.
"With your courage and your passion, we will realise our dreams," he told cheering participants.
Strike planned
He said the event was only the first step the campaign against the government and terrorism, promising to hold a further mass demonstration next week and to set a date for a nationwide general strike.
Other centrist-led unions which did not support the rally have joined the call for a strike.
The CGIL said the day's turnout exceeded a 1994 rally - also against Mr Berlusconi - when more than a million people voiced their opposition to his pension reform proposals.
Mr Berlusconi's government collapsed soon afterwards, and he was not voted back into power until last year.
Political tension in Italy has increased recently, with trade unions in particular voicing their discontent with the policies of Prime Minister Berlusconi.
It is now almost a year since his conservative government took over after six years of centre-left rule.
Mr Berlusconi is adamant that neither the unions' stance or the murder of Mr Biagi will stop his plans for labour reform.
He has invited his opponents in the labour movement for negotiations on Tuesday.