“I believe I’m going to cross the fence, even if they shoot me or cut me in half,” said Bilal Abu Zaher, 26, who came to the protest on crutches. He said he had been disabled since his house was damaged by an Israeli airstrike in the 2008 Israel-Gaza war. On Thursday, he said, Israeli soldiers shot at his wheelchair, damaging it. On Friday, he was back at the fence.

“I’m here for dignity,” he said. “My goal is to return to the land.”

The protests are aimed at Israel’s blockade of Gaza, which began after Hamas seized control in 2007. Billed as a six-week “March of Return,” the demonstrations are to culminate on May 15 with Nakba Day, which commemorates the flight and expulsion of hundreds of thousands of Palestinians during Israel’s 1948 war for independence.

By Friday, it was clear that they had already achieved one Palestinian aim: shifting the focus of international attention away from the struggle between Hamas and the West Bank-based Palestinian Authority — and onto the stark image of Gaza as a prison with Israel as its jailer.

Palestinian protesters flocked to the barrier fence on Friday, chanting, waving flags, flying kites — and setting fire to piles of tires.

Hamas, which has been effectively running the protests, said the smoke screen was a defensive measure to protect unarmed Palestinians from being shot, as many were last Friday, the first day of the demonstrations.

Israeli officials insisted the smoke screens were meant to provide cover for militants trying to make it across the barrier fence to attack soldiers and Israeli civilians living in farming communities nearby.

Either way, the smoke screens appeared effective: From the Israeli side, a detachment of Israeli soldiers ensconced behind earthen berms near the Nahal Oz kibbutz could only occasionally glimpse the hundreds of protesters in the Shajaiye neighborhood of Gaza City through vast clouds of acrid black smoke.

The Israelis first tried dispersing the smoke with an industrial-sized fan, trucked in on trailers. When that did not immediately work, they began trying to douse the fires with water cannons, setting off cheers among the protesters when that, too, did little.

In Shejaiya, one of five main gathering points for the protesters, ambulances and stretchers came and went throughout the day. At one point, as a lightly wounded man holding a slingshot was carried off, he made an obscene gesture toward the Israeli side, prompting cheers and laughter; another held up two fingers in a victory sign.

Israeli news outlets reported that Palestinians had built an ancient war machine — a trebuchet, or slingshot-like catapult — to hurl heavy stones or even burning tires at the Israeli side. The reports included photos of the trebuchet but there was no confirmation on Friday that it had been deployed.

The United Nations urged both sides to avoid violence and said there were “strong indications” that Israeli security forces had used excessive force last week in driving back protesters, which may have breached international law.

Elizabeth Throssell, a spokeswoman for the United Nations human rights office in Geneva, said the high casualty toll among protesters who were unarmed or did not pose a serious threat to security forces — and in some cases were running away from the fence — suggested Israeli troops may have used excessive force.

Some protesters had reportedly acted dangerously, but international law permits the use of lethal force only as a last resort in the face of an imminent threat of death or serious injury, she said. “An attempt to approach or cross the green-line fence by itself certainly does not amount to a threat to life or serious injury that would justify the use of live ammunition,” she said.