Israel on Saturday threatened to carry out military operations inside Gaza, a day after its soldiers killed 15 Palestinians and injured hundreds of others during a demonstration.
Israeli army spokesman Brig Gen Ronen Manelis said the gathering of an estimated 30,000 Palestinians along the Israel-Gaza border on Friday was not a protest but "organised terrorist activity" by Hamas, the group ruling Gaza.
"If it continues, we shall have no choice but to respond inside the Gaza Strip against terrorist targets which we understand to be behind these events," he said.
At least another 35 people were wounded in Israeli fire near the border on Saturday, the health ministry in Gaza said.
Meanwhile, thousands marched through the streets of Gaza at the funerals of those killed on Friday, while doctors at Gaza's Shifa Hospital operated on some of the 750 people health officials said were injured by live rounds.
Palestinian President Mahmoud Abbas declared a day of national mourning and shops in the West Bank were shuttered as residents observed a general strike ordered to protest against the killings.
The death toll was the highest in a single day since the 2014 Gaza war, the most recent of three wars fought between Hamas and Israel. The armed wing of Hamas said five of its members were among the dead on Friday.
"If they are going to continue with more killing, then we cannot tolerate such aggression forever," senior Hamas leader Mahmoud Zahar told The National.
He said it was a matter of "logic" that if more Palestinians were killed, there would be a response.
An estimated 30,000 Palestinians had gathered at five points near the Gaza-Israel border on Friday to begin a six-week protest endorsed by Hamas to highlight the right of refugees to return to homes in what became Israel.
Hamas official Ghazi Hamad said the march had sent a message to the world that Palestinians would not accept an American peace plan being formulated which is expected to abandon refugees and try to impose Israeli conditions on them.
"There is no place for an initiative that harms our national rights. We paid a price, but all the time we are sacrificing and we will continue to do so until we achieve our national goals."
The protest is to end on May 15, the 70th anniversary of Israel's establishment, which Palestinians observe as the Nakba, or catastrophe of displacement, when 700,000 people were expelled or fled, becoming refugees.
While organisers had called for Friday's march to be peaceful, smaller groups rushed toward the fence, throwing stones and rolling burning tyres. Israeli troops army responded with tear gas, rubber bullets and live fire. The army claimed firebombs were thrown at troops and that in at least one case gunfire was used.
Brig Gen Manelis denied allegations of excessive force, saying those killed were men aged 18 to 30 who were involved in violence and belonged to militant factions.
On Saturday, Palestinians released a video showing a young man collapsing after being shot while he had his back to soldiers and was moving away from the border fence. The Israeli army spokesman said in response that Hamas distributes films that show only parts of incidents and are "edited and fabricated".
UN Secretary-General Antonio Guterres has called for an independent investigation of the Israeli army's conduct, but the United States shielded Israel from any censure by the Security Council at an emergency meeting on Friday evening.
The Israeli human rights group B'Tselem said the army's actions were "illegal".
"Shooting at unarmed demonstrators is illegal and any command allowing such an action is manifestly illegal," B'Tselem said.
"Israel is trying to portray this as warfare and a war zone. But it isn't really fighting between armed soldiers and demonstrators. It looks like some people were shot in the back and at long distances while they were going away from the fence," said the group's spokesman Amit Gilutz.
Meanwhile, Palestinian search and rescue teams were being denied access to two Palestinians who were either dead or injured from Friday's gunfire and were lying 150 metres from the fence in an area that is part of a 300 metre zone Israel prevents Palestinians from entering, the Haifa-based Arab rights group Adalah said.
"The Israeli military continues to prevent Palestinian civil defence and medical teams from entering the area to evacuate the two men," Adalah said. The army said it was checking the matter.
Despite the mourning in the crowded coastal enclave, Hamas was seen as having been strengthened by the tragic events, with popular anger focused on Israel instead of on the inability of Hamas to stop a deterioration in daily living conditions.
"Hamas is the biggest winner," said Mkhaimar Abusada, a political scientist at Al Azhar University in Gaza City. "It succeeded in diverting attention from poverty, unemployment, closure and lack of electricity."
Before the protest, Hamas was under heightened pressure after President Abbas, who blamed the organisation for an assassination attempt against Palestinian Authority Prime Minister Rami Hamdallah, announced he had decided to take further "legal, national and financial" measures against Hamas.
Last year, Mr Abbas took steps he said were aimed at Hamas but which hit the civilian population hard. These included reducing salaries to civil servants and cutting payments to Israel for electricity supplied to Gaza.
Now, after Friday's events, Mr Abbas will not move ahead with the sanctions, his adviser Nabil Shaath told The National. "There is no chance he will take any measures that would hurt the Gazans," Mr Shaath said.
He described Friday's demonstration as "a real non-violent uprising the likes of which we have not seen since 1987" when the first intifada broke out.
"If this non-violent method of resistance continues and Israel continues with its brutal military force, it will make a major change in world opinion towards the occupation."