A crowd of at least 1,500 Palestinian activists streamed into central Gaza City to rally for national unity on Monday, a day ahead of schedule.
The activists marched to the Square of the Unknown Soldier, and chanted "The people want to end the division," in reference to the bitter split between Gaza's Hamas rulers and the Fatah-dominated Palestinian Authority in the West Bank.
Many brought tents and mattresses, while others were setting up water stations, with the intention of remaining in the square overnight until the official start of the demonstration on Tuesday.
"We are starting our sit-in now," organiser Ahmad Arar told AFP.
Some activists said the decision to start the mass protests a day early was taken for fear the Hamas-run security forces would close off the square or block roads in a bid to prevent the planned rallies.
The March 15 demonstrations are scheduled to take place simultaneously in Ramallah and Gaza City in a campaign put together on Facebook by a loose coalition of young activists, who say they have no political affiliation.
Similar protests are also planned outside Palestinian delegations overseas, in a move inspired by the recent wave of uprisings in the Arab world, which brought down the regimes of Egypt and Tunisia.
Earlier on Monday, Hamas prime minister Ismail Haniya had ordered the interior ministry to let the rallies go ahead without interruption, his office said in a statement.
The premier, it said, "supports all the efforts by the young people and the factions which aims to end the division and protecting the... national unity of our people."
Haniya ordered the ministry to ensure the security forces "created an atmosphere which would let these public events succeed."
Since the outbreak of massive protests in Egypt, Hamas security forces have clamped down on numerous demonstrations. Last week, they detained 11 "unity" protesters and interrogated those believed to be behind the Facebook campaign.