In Ramallah, the political capital of the West Bank, thousands of people were heading for the city centre, waving flags and chanting slogans, for a rally expected to start late morning to coincide wit the campaign be the 194th UN member state., an AFP correspondent said.
Other major rallies were planned in Nablus, Hebron and Bethlehem.
The demonstrations are part of the build up to Friday when Palestinian president Mahmud Abbas is expected to present to UN chief Ban Ki-moon a request for UN membership for a state of Palestine on the lines which existed before the 1967 Six Day War.
Palestinian officials have repeatedly pledged that the marches and demonstrations will be peaceful and stay within Palestinian-controlled areas, although Israeli forces have been preparing for months for the possibility they could turn violent.
Palestinian schools and government offices were to close to coincide with the start of the rallies, officials said.
In Ramallah, the main streets were cordoned off to traffic and a handful of policemen could be seen standing at a checkpoint on the city outskirts, watching as people poured onto the streets, the correspondent said.
Manara Square in the city centre was draped with a huge white flag bearing the words: "National Campaign for Palestine: State 194" as demonstrators shouted: "The people want the liberation of Palestine!"
Abbas is to submit the formal request for UN membership after he addresses the General Assembly on Friday in a move which is strongly opposed by Israel and the United States, which has vowed to block the membership bid in the Security Council.