The gathering came ahead of more nationwide protests Saturday by the non-partisan "You Stink" group, which has organised a series of growing protests in recent weeks against Lebanon's political class.
Supporters of the Free Patriotic Movement descended on Martyr's Square, carrying their party's trademark orange flags, as well the flags of allied Shiite movement Hezbollah.
FPM head Michel Aoun had urged supporters to demonstrate to demand parliamentary elections and a new electoral law that would see the public elect the president.
In Lebanon, the parliament elects the president -- but political divisions have prevented it from choosing a successor to Michel Suleiman since his mandate expired in May 2014.
"The people want the election of the president!" the demonstrators chanted.
Artists, political figures, and activists took to a makeshift stage to express support for Aoun and Hezbollah head Hassan Nasrallah.
"The future is for us, not for others," said Foreign Minister Gebran Bassil, a leading FPM figure.
"We want electricity and water, oil and gas. We want an electoral law that represents us and we want to elect a president!" Bassil called out.
Aoun made a brief appearance by way of a video link-up from an undisclosed location to tell protesters he was "proud of them."
- 'Outrage' at corruption -
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The FPM demonstration came as "You Stink" called for a nationwide mobilisation against a government they say is too corrupt to function.
"The people's outrage at this corrupt system continues... The protests will go on today and tomorrow in all Lebanese regions," the group wrote on its Facebook page.
The collective had called for demonstrations Friday in the coastal city of Tyre and in Zrariyeh, both in southern Lebanon, but an AFP correspondent in the area there said there were none.
For Saturday, activists have urged supporters to take to the streets in the eastern city of Chtaura, the historic town of Beiteddine and Nabatiyeh and Marjayoun in the south.
The protest movement began over a rubbish crisis that left pungent garbage piling up in Beirut and its outskirts, but it has evolved into a broad-based movement against government impotence and corruption.
Demonstrations organised by "You Stink" have escalated over the past two weeks, peaking Saturday when tens of thousands flooded Martyrs Square in a rare display of non-partisan mobilisation.
The speaker of parliament, Nabih Berri, has called for a national dialogue to discuss the paralysis plaguing Lebanon's institutions.
But "You Stink", which insists that its campaign for a political overhaul exempts no politicians, has accused Berri of diverting attention from their campaign.
On Thursday, 13 "You Stink" activists began a hunger strike that they said would not end until Mohammed Mashnuq resigned as environment minister.
In addition to his resignation, the campaign is demanding a lasting waste management plan, parliamentary elections and accountability for violence against protesters.
Rubbish has been piling up on the streets of Beirut and in the heavily populated Mount Lebanon area since the country's largest landfill closed on July 17.
Last week, the cabinet said several waste management companies had qualified as potential contractors but then said it would re-examine their bids.