Palestinian representatives have put an impassioned case to the World Court against Israel's West Bank barrier.
Palestinians took part in a "day of rage" against the barrier
They argued that the barrier - part fence, part wall - made the creation of a viable Palestinian state impossible.
Israel says the wall is necessary to prevent suicide bombers from killing civilians, and argues that the hearing could undermine the current peace plan.
Protests erupted along the barrier, with Israeli troops firing tear gas at stone-throwing Palestinians.
The United Nations General Assembly asked the International Court of Justice to consider the barrier issue.
The court ruling - if one is made - is not expected for several months and will not be binding. However, correspondents say it would have great symbolic importance.
The hearings, due to last three days, come after a Palestinian suicide bomber on Sunday killed eight people and injured dozens in an explosion on a packed Jerusalem bus.
KEY PLAYERS' POSITIONS
Palestinians: Barrier a violation of rights; crosses into occupied land, therefore illegal
Israel: Barrier vital for security, no political significance; ICJ has no jurisdiction; hearing could undermine peace moves
US: ICJ has no place in dispute; road map is way forward
EU: Against barrier crossing into West Bank; ICJ hearing divisive, inappropriate
44 UN members submitted arguments to court
Speaking for Palestinian case: 14 countries and organisations including South Africa, Arab League
The widow of a grocer killed in the bombing pleaded with the court not to rule against the barrier.
"I am appealing to you as someone... whose tragedy could have been averted by the separation fence," Fanny Haim wrote in the Israeli daily Yediot Ahronot.
The permanent Palestinian observer to the UN, Nasser al-Kidwa, made the opening statement to the court.
"This wall is not about security. It is about entrenching the occupation and the de facto annexation of large areas of the Palestinian land," he said.
"This wall, if completed, will leave the Palestinian people with only half of the West Bank within isolated, non-contiguous, walled enclaves," he said, arguing that the barrier would make it virtually impossible to establish a viable Palestinian state.
Israel says the barrier will help stop bombers killing Israelis
He condemned suicide bombings, but said Israel itself was ultimately responsible for them.
Mr Kidwa earlier told the BBC that Israel could have avoided accusations of stealing land if it had confined the barrier to its own territory.
The court is hearing presentations from 14 countries and organisations, all sympathetic to the Palestinian argument.
South Africa asked the court to rule against the barrier, just as the same tribunal ruled in 1971 that apartheid South Africa's presence in Namibia was illegal.
"South Africa, which was once a subject of this court, is in the midst of celebrating 10 years of our democracy," Deputy Foreign Minister Aziz Pahad said.
"After centuries of division and conflict, South Africans found the political will to build a new democratic society."
Israel argues that the court has no jurisdiction to rule on what it considers a political - not legal - dispute with the Palestinians.
Several dozen countries, including the US and many European Union nations, have backed that view.
Israel also says a ruling on the barrier could undermine the Middle East peace plan known as the roadmap.
Israel made a written submission to make its point, but is not making oral arguments in response to the Palestinian case.
The hearing was one-sided and failed to address the question of Palestinian terrorism, the Israeli delegation said in a statement.
"It seeks to put Israeli's measures to stop terrorism on trial but not the terrorists themselves," the statement said.
The Israelis, while not sending a legal team, flew in a bus to The Hague that was destroyed by a suicide bomber in Jerusalem on 29 January.
Dutch police kept pro-Israeli and pro-Palestinian demonstrators rigorously separated.
In the West Bank, Palestinian protesters in the West Bank were spurred on by Yasser Arafat, who in a televised speech urged them to "make their voices heard against this wall of expansion and annexation".
In Abu Dis, demonstrators threw stones at Israeli police who responded with tear gas.
There were also protests in most other West Bank towns, including Tulkarm, Nablus, Jenin and Hebron, and in the Gaza Strip.
When it is finished, the barrier will be 700km (440 miles) long.
Most of it consists of electronically monitored fences and exclusion zones. But in some sections it is a high concrete wall.
Hundreds of Israelis have been killed in suicide bombings since the Palestinian uprising began more than three years ago.
More than 2,700 Palestinians have been killed during the same period.