The rally was called by Lebanon's opposition which is demanding an end to Syrian interference in the country's political affairs and a withdrawal of its estimated 14,000 troops.
Waving the red and white scarves of what the opposition is calling its "peaceful uprising for independence," demonstrators shouted "Syria out" and "Down with the government" as they marched on the site where al-Hariri was killed on 14 February.
Lebanon's government, facing mounting calls to resign over the murder, has vowed to cooperate with UN investigators to find his killers but rejected a full international inquiry.
Security was tight for the demonstration with anti-riot police and soldiers out in force in Beirut, while lawmakers were also preparing for a stormy session in parliament, the first since the murder.
Lebanese journalist Cilina Nasser told Aljazeera.net that the demonstration was peaceful and security forces did not interfere.
"Usually if there is a anti-Syrian protest the security forces quickly want to crush it. But this time they didn’t try to disperse it," she said.
Security forces did not intervene
in the protest
She said that the protest included many different elements.
"Left wing and right wing, Muslims and Christians have come together to demand a withdrawal," she said.
The rally includes a five-minute period of silence at 1055 GMT, the exact time of the massive bomb blast that tore through al-Hariri's motorcade on the Beirut seafront last Monday, killing 17 people according to media reports.
Opposition figures criticised the government for the heavy deployment of security forces and the Lebanese army on the streets of Beirut during the protest.
Government authorities have justified military deployment citing security reasons.
The authorities say it is their responsibility to maintain law and order especially with tensions running high and it is in the interest of the public to maintain security.
The late al-Hariri has been credited
for Lebanon's post-war rebuilding
al-Hariri, 60, a five-time prime minister and a billionaire tycoon credited with spearheading Lebanon's post-war rebuilding, had quit as premier in October in a row over Syrian interference.
In Beirut, Lebanese security forces were on alert to guard against the opposition demonstration turning violent, while some schools in the capital have been closed.
"What I fear in this tense situation, any foreign party could infiltrate into the ranks of security forces or the students to carry out any act," Interior Minister Sulaiman Frangiyah said.
People continue to converge each evening on Martyrs' Square, where al-Hariri is buried, to shout "Syria Out" and "Down with the government," despite warnings from the authorities that they will start banning illegal gatherings.
The opposition has been galvanised by al-Hariri's killing into piling on the pressure for Syria to pull out its troops and intelligence operatives and end its political domination of Lebanon.
It succeeded on Monday in securing an agreement from Lebanon's parliamentary speaker Nabil Barri for a debate on the situation.
A meeting of Lebanese political leaders on Sunday had rejected calls for the government to quit and said the crisis should be resolved through dialogue.
The opposition has been
galvanised by al-Hariri's killing
Opposition MPs, who make up about a third of the 128-seat parliament, have rejected any talks before al-Hariri's killers are found.
Druze MP Walid Jumblatt, a leading opposition figure who says he has fears for his own personal safety, has said that the only direct dialogue with Damascus should be held to oversee a Syrian withdrawal.
Damascus blames Israel
The Lebanese government denies any role in the al-Hariri killing and Damascus has accused Israel, denouncing what it calls "campaigns of intimidation" against it by the United States and the Jewish state.
Lebanon said on Sunday it would cooperate with a UN commission of inquiry into the killing to find those responsible and try to ease tensions provoked by the murder although it rejects a full-blown international probe.
Internationally, France and the United States, which co-sponsored a UN Security Council resolution last September demanding the withdrawal of foreign troops from Lebanon, have been the most vocal in insisting on a UN inquiry.
Arab League Secretary-General
Amr Musa arrived in Syria for talks
A UN team is due to arrive in Lebanon this week, headed by Ireland's deputy police commissioner Peter Fitzgerald.
Amid the deteriorating situation in Lebanon, Arab League chief Amr Musa is visiting an increasingly isolated Syria, where he will hold talks on Monday with President Bashar al-Asad and Foreign Minister Faruq al-Shara.