As many as two million workers from all over Italy converged upon Rome for a mass demonstration.

Originally intended to focus attention on opposition to Berlusconi's plans to relax labour law, the demonstration has now widened to a protest against terrorism after the killing of Marco Biagi, one of the architects of the proposed reforms.

Biagi (photo), a respected economist who had worked with both the former centre-left government and the current conservative one, was shot dead in front of his home on Tuesday as he bicycled home from work.

The labour reforms that he had drafted would make it easier to appoint and dismiss workers.

CGIL storms Rome

Traffic in congested Rome came to a standstill after hordes of members of Italy's largest union, CGIL arrived in the heart of the city by about 9,000 buses and 60 special trains.

A sea of red CGIL flags made its way through Rome amid heavy police presence and hundreds of anti-globalisation protestors.

"We are many and we are here to join the fight against terrorism and to defend our rights", CGIL leader Sergio Cofferati said before he was to address the massive crowds which gathered in the huge Circo Massimo, site of ancient Roman chariot races.

"Saturday's demonstration will no longer have a party atmosphere. After the killing of Marco Biagi, the priorities and order of the day will inevitably change", Cofferati was reported as saying.

The shooting of Biagi has rekindled fears of a return to the politically motivated violence that scarred Italy in the 1970s and 1980s.

At that time the Red Brigades had carried out a series of assassinations. Responsibility for Biagi's murder has been claimed by an organisation linked to the Red Brigades.

Trade unions in Italy are increasingly disgruntled with the policies of Prime Minister Berlusconi. It is now almost a year since his conservative government took over the country after six years of centre-left rule.

Berlusconi is insistent that the murder of Biagi will not stop his plans for labour reform.

In a televised address he said, "the terrorists must know that they will not stop reforms, not stop change, not stop the work of the government or the coalition".

"Unfortunately in Italy it is always those who fight for reform and change that get attacked".

Saturday's protest march is one of several planned by unions in the coming days and weeks.