JERUSALEM >> What was billed as a six-week campaign of peaceful protests in Gaza descended almost immediately into chaos and bloodshed today, with health officials in the Palestinian territory saying Israeli soldiers killed 15 Palestinians in confrontations along the border fence.

Soon after the campaign began this morning, the Israeli military said Palestinian protesters were rioting in six places along the border, rolling burning tires and hurling stones at the fence and at Israeli soldiers beyond it.

Later, the military said that Palestinians threw Molotov cocktails at soldiers and that it had also thwarted a shooting attack in northern Gaza and an attempt to infiltrate the border. Israeli forces fired live ammunition, rubber bullets and tear gas.

By nightfall, when the activity near the border fence appeared to have subsided, the Palestinian Health Ministry said 15 Palestinians had been killed by Israeli fire and more than 1,000 injured.

Mahmoud Abbas, president of the Palestinian Authority, declared Saturday a national day of mourning.

The Israeli military said it was not able to verify the death toll.

Declaring the area surrounding the Gaza Strip a closed military zone, the Israeli military said it had been responding with riot control methods and had been shooting toward the “main instigators.”

The protest came at a particularly charged time, as Jews prepared for the start of the Passover holiday Friday evening and Palestinians observed Land Day. The day commemorates the events of March 30, 1976, when Israeli security forces shot and killed six Arab citizens of Israel during protests over the government’s expropriation of Arab-owned land in northern Israel.

With Gaza’s economy collapsing, fears of an explosive response had been mounting. In recent years, first Egypt and then the Palestinian Authority, which administers the West Bank, joined Israel in squeezing Gaza financially.

The Palestinian organizers had called for peaceful protests, and men, women and children were bused to tent encampments that popped up in recent days about 700 yards from the border with Israel. The organizers intended for the six-week campaign to culminate in a mass march toward Israel, which had put Israeli officials on edge.

About 20,000 demonstrators had gathered at the tent encampments by Friday morning, but most appeared to have remained well away from the border fence and did not engage in violence.

They were protesting Israel’s more than decadelong blockade of Gaza, which restricts the movement of people and goods in and out of the Palestinian territory and which Israel calls a security imperative. The campaign was also intended to highlight Palestinian demands for the right of return to the lands that became Israel 70 years ago. A majority of Gaza’s 2 million residents are either refugees of the 1948 war that broke out over Israel’s creation or descendants of those refugees.

Girding for violence, Israel had almost doubled its forces along the border, deploying snipers, special units and drones and warning that it would act to prevent any breach of the border fence or violation of its sovereignty.

The idea for the border encampments, in about half a dozen locations, was initiated by a Gazan social-media activist, Ahmed Abu Artema, a political independent. It was adopted by Hamas, the Islamic militant group that controls Gaza and is known for its armed resistance. Hamas promoted the protest on its social media platforms and urged Palestinians to participate.

“Our will in achieving the actual return to our lands is more powerful than jet fighters and a gun,” Abu Artema said by phone Friday as he was on his way to the protest. “This march is rightful and will not be used and exploited for political agendas.”

For Israel, the prospect of unarmed mass protests posed the challenge of trying to maintain deterrence by threatening harsh measures while also trying to avoid significant civilian casualties. B’Tselem, an Israeli human rights organization, warned that any shoot-to-kill policy against unarmed demonstrators would be illegal unless the soldiers’ lives were threatened.

Before the larger clashes broke out, the Palestinian Health Ministry in Gaza reported that a Palestinian man, a farmer, was killed near the border zone early Friday by Israeli artillery fire — one of the 15 it reported dead later in the day. The Israeli military’s account said one of its tanks had fired on two Palestinians who approached the border and were “acting suspiciously.”

“We are raising the flags of peace and have nothing to harm the enemy,” said Hamed Jundiya, 63, an educational supervisor who erected his tent a few hundred yards from the border fence. Gazans are desperate, he said, “living without work, electricity and open borders.”

Another protester, Mohammad Obaid, 18, took a more militant stand, saying that holding a Palestinian flag in one hand and a rock in the other would be enough to get him killed by an Israeli soldier. “We can bring back our lands with the power of guns and weapons, not with a march, a stone or a knife,” he said.

Even before the protests started, Israel had begun a campaign to hold Hamas responsible for any violence. Israel’s hard-line defense minister, Avigdor Lieberman, warned Gazans to keep away from the border in a post on Twitter written in Arabic.

“The Hamas leadership is risking your lives,” he wrote. “I advise you to get on with your normal everyday lives and not to participate in the provocation.”

The peak of the Gaza protest was supposed to take place May 15, when Palestinians commemorate what they call the Nakba, or catastrophe, the anniversary of Israel’s declaration of independence and the 1948 war in which hundreds of thousands of Palestinians lost their homes through flight and expulsion.

This year, May 15 is expected to be particularly explosive. It comes a day after the planned move of the U.S. Embassy to Jerusalem from Tel Aviv, a step that has provoked international criticism and Palestinian outrage. It also coincides with the start of Ramadan, the Muslim holy fasting month.

Organizers of the border protest had hoped to create an almost festival-like atmosphere to attract families, setting up portable washrooms and providing free food, water and Wi-Fi.

But tensions in Gaza had been building for weeks. Palestinian militants planted explosives along the border and some cut through the border fence, armed with knives and grenades, and set fire to Israeli military equipment — apparently testing Israeli preparedness and putting local communities on edge.

The protest fed on Palestinian anger over the long blockade and the failing reconciliation process between Hamas and Fatah, the rival, mainstream movement led by Abbas, whose Western-backed Palestinian Authority holds sway in parts of the West Bank.

Abbas, whose forces were routed from Gaza during factional violence in 2007, has vowed to tighten economic sanctions on the enclave, where most of the population lives in poverty and lacks such basics as regular electricity.

Israel has fought three wars in Gaza over the past decade and has invested heavily in combating the threat posed by rockets fired by Hamas and other militant groups, and from tunnels crossing under the border.