It is one of the biggest anti-government protests in years

The protesters, led by opposition figures, shouted slogans blaming Syria for the killing.

Security forces have closed off most of the capital's central area and set up checkpoints on approaches to the city.

A BBC correspondent says it is one of Lebanon's biggest public protests for years. Many companies and schools have shut for the day to let people attend.

Correspondents say Hariri's assassination has heightened tension over Syria's political and military sway in Lebanon and has to international pressure for Syria to pull out of its smaller neighbour.

'Independence uprising'

Police stopped buses from reaching the capital from the north and east but most protesters continued on foot.

The opposition observed a five-minute silence at 1055 GMT, the moment of the blast that killed Hariri and some 15 other people a week ago.

It is the 21st century, and people should be able to govern themselves. We have to regain our country Anti-Syria protester

At the same time there was also an official one-minute's silence for the former prime minister, with TV and radio stations broadcasting the national anthem.

Many protesters out on the streets wore red and white scarves, symbolising the opposition's "independence uprising", which it describes as a peaceful campaign to dislodge the pro-Syrian Lebanese government and force out the 15,000 troops Syria keeps in Lebanon.

"It is my civic duty as a Lebanese to take part in this uprising," said one protester Youssef Mukhtar, quoted by the Associated Press news agency.

"Enough bloodshed and disasters. It is the 21st Century, and people should be able to govern themselves. The situation has become unbearable and we have to regain our country," he added.

War of words

Stepping up pressure on Syria, US President George W Bush repeated his call for Syria to pull its troops out of Lebanon.

Speaking at the start of a five-day visit to Europe, Mr Bush said: "Just as the Syrian regime must take stronger action to stop those who support violence and subversion in Iraq - and must end its support for terrorist groups seeking to destroy the hope of peace between Israelis and Palestinians - Syria must also end its occupation of Lebanon."

The European Union has added to a call by the US for an international investigation into the killing in a massive car bomb last Monday.

Police have prevented all but pedestrians from reaching the rally

"There has to be an independent probe, given a high-level of suspicion about the potential involvement of Syria in the assassination of former Prime Minister Hariri," said UK Foreign Secretary Jack Straw.

Lebanon's Syrian-backed government says it will co-operate with a UN investigation team due to arrive this week, but has rejected calls for a full international inquiry.

Arab League head Amr Moussa - who has been holding talks in Damascus to ease the tension - said Syria would soon take steps towards withdrawing troops from Lebanon.

"President Assad stressed more than once his firm determination to go on with implementing the Taif agreement and achieve Syrian withdrawal from Lebanon, in accordance with this agreement," Mr Moussa said, referring to the 1989 deal which brought the civil war to an end.

But in an interview with the BBC, Syrian Information Minister Mahdi Dakhlallah said Mr Moussa had misunderstood the Syrian leader.

Mr Dakhlallah said Syria's position was that there would only be a redeployment of Syrian forces within Lebanon.

Hariri - who, with Syria's backing, had served as prime minister for 10 of the last 12 years - resigned in October in a row over Syrian interference.