Armenians say some 1.5 million of their people were systematically killed by authorities between 1915 and 1917, in the dying days of the Ottoman empire.
Some 20 countries have recognised that as genocide, but Turkey refuses to do so, saying civil war and famine led to the deaths of 300,000 to 500,000 Armenians and as many Turks.
Thousands of people, some holding signs reading "100 years of impunity" and "Criminal Turkey," marched from the Armenian Catholic church in Antelias, north of Beirut, to the suburb of Burj Hammoud, a stronghold of the country's Armenian minority.
Other posters reading "Armenian Genocide 2015: truth will triumph," and "Never forget, never forgive" floated above the sea of people marching along the Mediterranean coastal highway, flanked by soldiers and police.
Young marchers adorned their hair and T-shirts with forget-me-nots, lavender flowers with bright yellow centres that have become the symbol of this year's remembrance.
"My grandfather was four years old when he escaped the genocide," Rafi Isaakian, 69, told AFP. "We are here to remember the 1.5 million Armenians that perished."
Signup to our newsletter and follow us on Facebook and Twitter!
"I tell my grandchildren to never forget the Armenian cause. We need to reclaim our rights, and (the Turks) should recognise the genocide," said another man who marched, like many others, with his children.
Education Minister Elias Abu Saab, a Greek Orthodox Christian, had ordered schools closed across the country in commemoration of the event, which sparked anger among some Muslim and Islamist communities.
In the Muslim-majority northern port of Tripoli, Turkish flags were hung in storefronts, outside of homes and even over cars, especially in the city's old souks.
The head of the Lebanese Turkish Friendship Association, Zaher Sultan, told AFP his organisation had distributed 10,000 Turkish flags throughout the city "in solidarity with Turkey and in response to the fabricated accusations against it."
The office of Tripoli's mufti criticised the school closures, calling the issue of genocide "controversial" and saying Lebanon should not "choose sides."
Lebanon's Armenian community, many of whom are descendants of survivors, makes up about four percent of the country's population.