In Monday's rally, demonstrators directed barbed jokes at Syria - a picture of Syrian President Bashar al-Asad looking ruefully at his late father, Hafidh al-Asad, read: "Papa don't preach, I'm in deep trouble."
Addressing the crowd, Bahiya al-Hariri, a Lebanese opposition MP and assassinated former prime minister Rafiq al-Hariri's sister, demanded that an international investigation be undertaken and that Lebanese security chiefs be sacked.
The rally was the largest broad-based demonstration the opposition has held in its series of protests demanding Syria remove its forces from Lebanon since the assassination of al-Hariri.
Druze rolled in from the mountains east and southeast of Beirut, Christians from the heartland in the northeast, and Sunni Muslims turned out in large numbers from the north.
Protesters who came in from Lebanon's regions could be picked out by their distinctive accents.
A powerful turnout was key for the opposition movement, whose credibility was on the line after the Shia group Hizb Allah last week brought out an estimated half-million people in support of Syria, and after the Lebanese government re-appointed Prime Minister Umar Karami.
Karami had resigned - the opposition's most concrete accomplishment - after the first demonstrations protesting against al-Hariri's assassination and the presence of Syrian troops in Lebanon.
"The UN investigators have become convinced that there was a cover-up of evidence at the very highest levels of the Lebanese and Syrian intelligence authorities"
British daily The Independent, on al-Hariri's assassination
The number of protesters on Monday was estimated to have reached a million, according to the Lebanese LBC TV. Others have put the figure at about 800,000.
The coastal highway from northern Lebanon was turned into a one-way street, jammed with cars and buses bringing in protesters.
Flags fluttered from windows plastered with pictures of al-Hariri.
Some people were stuck in their cars for up to six hours while others left their vehicles and walked.
Some even raced on speedboats to the seaside Martyrs' Square. Along with TV advertisements, the opposition sent out a flurry of email and telephone text messages calling on people to show up.
Text messages urged protesters
to converge on Martyrs' Square
One message pointed to Karami's claim that the Hizb Allah rally had shown that the government had the support of the majority of Lebanon's estimated four million population.
"Prove him wrong by being at Martyrs' Square," the call flashed across cell phones and computers.
Members of parliament called for an international investigation into al-Hariri's killing.
Another MP, Marwan Hamada, asked protesters: "You want the truth on the assassination?"
They answered the question by saying: "It's lying in the dark chambers of the intelligence services that are ruling us and that you are in the process of sweeping out."
His comments came as British newspaper The Independent wrote that a United Nations investigation into al-Hariri's killing would report that Lebanese and Syrian authorities were behind a cover-up of evidence from the 14 February blast.
"The UN investigators have become convinced that there was a cover-up of evidence at the very highest levels of the Lebanese and Syrian intelligence authorities," the daily said.