By Jim Muir
BBC News, Beirut
From shortly after dawn they began streaming in their hundreds through Beirut's southern suburbs, the heavily Shia area controlled by Hezbollah which was massively pounded by Israeli jets during last summer's war.
There were many women among the crowds in Beirut
By the time the ceremony got under way hours later, probably several hundred thousand Lebanese Shia - men, women and children, old and young, most of them dressed in black - had converged on the site chosen for the Ashura celebration.
Ashura celebrates the martyrdom in AD680 of the Imam Hussein, grandson of the prophet Muhammad.
His death at the hands of the Sunni Caliph Yezid was the defining moment that sealed the schism between Shia and Sunni Islam.
It is a schism very much in evidence in Lebanon more than 13 centuries later, especially after violent Sunni-Shia clashes on the southern edge of Beirut city on Thursday, which left four people dead and around 150 wounded.
Hezbollah and its Shia ally Amal are spearheading an unyielding opposition campaign to topple the Western-backed Beirut government, which is generally supported by the Sunnis.
Fears are now rising of a civil war in which one of the main components would be a devastating sectarian collision between the two branches of Islam.
Message of restraint
Although it is primarily a religious event, this year's Ashura inevitably took on added significance in the current politically-charged climate.
It was bigger, more organised, and more protected than in previous years, and it culminated in a fiery oration from the Hezbollah leader, Hassan Nasrallah.
The tight security was provided entirely by plain-clothes Hezbollah operatives, dressed in black and carrying walkie-talkies.
No guns were on display. Not a single Lebanese army soldier or policeman was to be seen.
The Hezbollah leader addressed crowds gathering for the festival
In all other parts of Beirut but the southern suburbs, the official security forces are in charge and much in evidence in key locations.
In this context, most observers saw the mobilised turnout as a show of demographic strength by Hezbollah and its supporters.
But it also carried a message of restraint, with Hassan Nasrallah and many of those who attended insisting that they would refuse to be drawn into civil war even if it meant ignoring fatal attacks and provocations against them.
"Shias don't want civil war, especially those who obey the Imam Hussein and Sheikh Nasrallah," said Tareq, a 30-year-old teacher, as the ceremonies came to a close.
"Civil war is killing Muslim with Muslim. I think after what Sheikh Nasrallah said, if they're going to kill Shias, the Shia are not going to respond. That's how it should be."
"Imam Hussein was killed, and by his sacrifice, we say that he continued Islam. If we are to continue with this nation, people must not kill each other, and if one is killed, the others must not respond."
'We must learn'
"It's America and Israel who want trouble here," said another participant.
"Their role is clear. In Palestine there are no sects, but there is trouble based on groups and parties. Here in Lebanon there are different sects, so they're working on fomenting sectarian clashes.
"But it won't work, because we insist on Islamic unity."
Although Ashura is primarily a Shia occasion, many of those taking part insisted that it was an important occasion for all Muslims, and should draw them together rather than exacerbating tensions between them.
"I believe this day should be one of cohesion among the Muslims," said one woman.
"I believe that Islam will certainly bind together again, and even if there are people who are trying to stir up dissent between us, nobody will accept.
"We must learn from Ashura that we must be understanding of one another and come together, so we certainly don't fear civil war."
Hezbollah leader Hassan Nasrallah has repeatedly stressed his determination not to get dragged into sectarian strife with other Lebanese factions, especially the Sunnis.
"Even if you shoot 1,000 of us, we will not shoot back," he said shortly before last Tuesday's general strike, which saw widespread violence in many areas.
While Hezbollah is recognised as highly organised and disciplined, the same cannot be said of many of its allies in the opposition movement, which includes some Christian and other factions.
Nor is discipline and control a trademark of many of the groups supporting the government.
Within the anti-opposition spectrum there are also a number of shadowy extremist Sunni groups in the mould of al-Qaeda.
Some of the rituals of Ashura can be dramatic and bloody
There is much talk, too, of a "fifth column" busily engaged in stirring up trouble by inserting snipers and agitators into hot spots.
While Hezbollah may have sworn to refrain from sectarian strife and violence, it has also insisted it will keep up the pressure for a share of government big enough to give the opposition veto power, and for early general elections which it hopes would sweep the current government away.
With tensions running so high, nobody knows what the future holds in store, even a day or two ahead, far less in the medium and long term.
"We just don't know what's going to happen," said one prominent business figure and political analyst.
"It's impossible for us to plan anything at all when it comes to major decisions on business projects."