Rallies were held across Northern Ireland and the rest of the United Kingdom in support of the besieged Gaza Strip.
The Gaza Strip has been blockaded and bombarded by Israel after its Hamas rulers launched a series of terrorist raids on October 7.
More than 1,400 people in Israel have been killed, mostly civilians attacked in the October 7 incursion. Over 4,100 people have been killed in Gaza, according to the Hamas-run health ministry.
Ms O'Neill was present at a demonstration in Dungannon and she called on the Israeli government to "stop the occupation, illegal settlements and apartheid" in a post on the X platform, formerly known as Twitter.
She posted: "Today tens of thousands of people will take to the streets in towns and cities across Ireland in solidarity with the Palestinian people.
"I joined my local rally in Dungannon, and the message from there and across the island to the Israeli government is clear and simple - STOP.
"Stop the bombing of Gaza. Stop the blockade of Gaza. Stop the occupation, illegal settlements and apartheid.
"The international community must step up and call for a full, immediate ceasefire. This is not self-defence. It is oppression of an entire population.
"We need immediate ceasefires, full adherence to international law and urgent dialogue to reach a just and lasting peace."
However, a TUV spokesperson responded: "In the immediate aftermath of the brutal terrorist attack on Israel many senior Republicans nailed their colours to the mast even while the world was reeling from the images of bodies of civilians paraded in the streets of Gaza while other civilians were dragged off as hostages.
"Two weeks after this while the one who would be First Minister is happy to join white line protests against Israel's targeting of terrorists she still hasn't tweeted a word about those who remain held as hostages in Gaza."
Meanwhile, a large crowd attended a rally in Belfast which started at the City Hall and ended up outside the BBC’s headquarters in the city.
Roads were closed for the demonstration with a strong police presence.
There was also a rally in Londonderry which was attended by SDLP leader Colum Eastwood.