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Gunfire from Israeli soldiers across a border fence killed four Palestinians on Friday, including a 15-year-old boy, health officials confirmed to NBC News. More than 150 others were also wounded as several thousand people in blockaded Gaza staged a fourth round of weekly protests on the border with Israel.

Huge black plumes of smoke from burning tires engulfed the border area. Some of the activists threw stones toward the fence or flew kites with flaming rags dangling from their tails.

The latest deaths brought to 32 the number of Palestinians killed by Israeli troops in protests since late March. More than 1,600 have been wounded by live rounds in the past three weeks, according to the Gaza Health Ministry.

The rising Palestinian casualty toll signaled that Israel's military is sticking to its open-fire rules despite international criticism of the use of lethal force against unarmed protesters. Israel says it's defending its border, and alleges Gaza's ruling Hamas uses protests as cover for attacks.

Riyad Mansour, the Palestinian ambassador to the United Nations, said in a letter to the U.N. Security Council late Friday that Israeli forces continue demonstrating the "cruelty of their occupation machine, responding to the calls of unarmed civilians for freedom and justice with brutal and lethal force." He said one of the latest victims was a 25-year-old disabled man.

Nickolay Mladenov, the U.N. special coordinator for the long-stalled peace process between the two countries, said on Twitter Friday that the 15-year-old's death is "outrageous."

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🔴 It is OUTRAGEOUS to shoot at children! How does the killing of a child in #Gaza today help #peace? It doesn’t! It fuels anger and breeds more killing. #Children must be protected from #violence, not exposed to it, not killed! This tragic incident must be investigated. — Nickolay E. MLADENOV (@nmladenov) April 20, 2018

Turnout for the marches has fluctuated, with the biggest showing on March 30, but Friday's crowd appeared to have been somewhat larger than the one the previous week.

The marches are part of what organizers, led by Hamas, have billed as an escalating showdown with Israel, to culminate in a mass march on May 15.

The top Hamas leader, Ismail Haniyeh, said Friday that people should get ready for large crowds spilling across the border that day. "Our people will outnumber the occupation and force it from our land," he said, referring to Israel.

Hamas says the protests are aimed at breaking a crippling border blockade that was imposed by Israel and Egypt after the Islamic militant group overran Gaza in 2007, a year after winning Palestinian parliament elections.

The marches also press for a "right of return" of Palestinian refugees and their descendants to what is now Israel. Hundreds of thousands of Palestinians fled or were forced from homes in the 1948 war over Israel's creation. Palestinians mark May 15, the anniversary of Israel's founding, as their "nakba," or catastrophe, to mourn their mass uprooting.

Several thousand protesters flocked to the border area Friday, most gathering at five tent camps several hundred meters away from the border. Smaller groups advanced toward the fence, throwing stones, burning tires and flying kites with burning rags.

A Palestinian youth holds a kite with the colors of his national flag at the site of protest tents next to the Gaza Strip's border fence with Israel MOHAMMED ABED / AFP - Getty Images

The kites are part of a new tactic aimed at setting fields on the Israeli side on fire. Most kites showed the colors of the Palestinian flag. One white kite bore a Nazi swastika.

The Israel Defense Forces said in a statement that "The IDF will not allow any harm to security infrastructure that protects Israeli civilians, and will act against the violent rioters and terrorists who threaten either."

Earlier on Friday, Israeli military aircraft had dropped leaflets urging Palestinians to stay away from the fence and warning that they endanger their lives by following Hamas directives.

Meanwhile, Palestinians were using kites to drop leaflets on the Israeli side claiming Jerusalem is Palestine's capital and urging Israelis not to respond to their leadership.

While Hamas and smaller Palestinian factions have taken a lead as organizers, the mass marches are also fueled by growing desperation among Gaza's 2 million residents.

The border blockade has trapped nearly all of them in the tiny coastal territory, gutted the economy and deepened poverty. Gaza residents typically get fewer than five hours of electricity per day, while unemployment has soared above 40 percent.