The children had been playing on swings and buying snacks from a neighborhood market during a holiday lull in the fighting between Israel and Palestinian militants.
But the relative quiet ended Monday when explosions hit a refugee camp and just outside the nearby Shifa hospital, the main medical facility in the Gaza Strip.
Where the swings had stood, there were pools of blood. At least 10 people died, most of them children.
Palestinian officials and witnesses said Israeli airstrikes caused the blasts. Israel swiftly disavowed any responsibility, blaming failed rocket attacks launched by "Gaza terrorists."
On the Israeli side of the border, the military said four soldiers were killed by mortar rounds that struck a community close to Gaza. A fifth died in fighting in the coastal enclave, which is controlled by the Islamic militant group Hamas.
Adding to Israeli anxiety, officials and media reports said at least five militants from Gaza managed to cross into Israel using an infiltration tunnel near a kibbutz called Nahal Oz. Residents of the kibbutz, which is half-deserted because of previous rocket attacks, were told to stay inside and lock their doors, and roads in the area were closed. The Israeli military said its forces had killed one of the militants.
In addition, the military said Palestinian rockets were fired toward the northern city of Haifa, which had been out of range from Gaza.
With hopes fading for a speedy halt to the fighting, Israeli Prime Minister Benjamin Netanyahu declared after a Cabinet meeting that "we must be prepared for a prolonged campaign."
"We will continue to act aggressively and responsibly until the mission is completed to protect out citizens, soldiers and children," he said.
Palestinian militants remained defiant. "These threats do not scare Hamas or the Palestinian people, and the occupiers will pay for the massacres of children and civilians," Hamas spokesman Sami abu Zuhri said in a statement.
As dusk approached, the Israeli military sent out text and recorded phone messages warning people in five northern and eastern Gaza communities — Beit Hanoun, Beit Lahiya, Shajaiya, Zeitoun and Jabaliya — to leave their homes and move toward the center of Gaza City. Many of those in Jabaliya had been taking shelter there after fleeing previous bombardment in their own neighborhoods.
Soon after, heavy explosions illuminated the night sky.
The bloodshed followed a call by the United Nations Security Council for an immediate and unconditional cease-fire in the 3-week-old conflict, which has killed more than 1,050 Palestinians. According to U.N. estimates, most of the Palestinians killed were civilians. The fighting has also killed at least 48 Israeli soldiers and three civilians, including a foreign worker.
The United States and other mediators had hoped to forge a durable cease-fire before the start Monday of the three-day Eid al-Fitr holiday, one of the most important on the Muslim calendar, marking the end of the holy month of Ramadan. But efforts over the last week failed to bring the two sides together.
Encouraged by the relative calm that lasted most of the day, parents had allowed their children a few hours to play on swings and hand-turned Ferris wheels set up on neighborhood streets during the holiday.
The children killed in the Shati refugee camp had congregated around a swing set up in front of a small market, residents said. Ahmed Zaqout was standing across the street when two shells landed among them, and he rushed to help.
"One second they were playing on the swing, and the next their body parts were all over the place," said Zaqout, whose clothes were soaked in blood. "We didn't know whose leg or arm we were carrying."
Frantic parents streamed into the nearby Shifa hospital, cradling their bloodied children. A mother wailed, "My kids are gone. My kids are gone." Outside the hospital morgue, a seething crowd gathered to collect the bodies and bring them home for burial.
"There is no safety," said Abdullah Harara, 23. "They are bombing everything."
Another shell smashed through the wall of the hospital compound, scattering people who had been sheltering there since fleeing fighting elsewhere. At least three people suffered shrapnel wounds, hospital staff said.
Two friends returned later to collect bundles of clothes they had left behind in the panic. "I would rather go back and die in my house," said Ibrahim Desouqi, 23.
Israel says it makes every effort to avoid civilian casualties, issuing warnings when it intends to attack neighborhoods and homes. It blames Hamas for operating among civilians and putting them in harm's way.
Later that night, rockets streaked from the refugee camp toward Israel. Residents cheered.
Even on what had been a quiet day by recent standards, the crackle of gunfire and the thud of explosions could be heard in northern Gaza. Israeli shelling there killed two people, including a 4-year-old boy, local health officials said.
The Israeli military said mortar rounds were fired at its forces in several locations within Gaza and that a number of projectiles were directed at Israel.
In south-central Gaza, heavy exchanges of fire were reported between Israeli forces and Hamas militants in the vicinity of Khan Yunis. Palestinian rescue workers pulled several more bodies from the rubble in Khuza, a village east of Khan Yunis that has been a focus of the clashes in recent days.
Across Gaza, Eid commemorations were muted by the death and destruction of the last three weeks. With entire neighborhoods in ruins, a normally festive time was marked by fear and anxiety.
Faez Harazeen brought his family to a small park in Gaza City for a few hours' respite. Their home was destroyed in shelling last week, and they are now taking shelter at his sister's overcrowded home.
They spread a sheet under a tree. Nearby, children were lining up for rides on horses and in toy cars festooned with tinsel. Then a series of explosions shattered the afternoon calm.
"They follow us wherever we go," Harazeen cried as families scattered.
At a school-turned-refugee center in Beit Lahiya in northern Gaza, displaced Palestinian families spilled out of classrooms and open-air corridors. Clothes hung from windowsills and railings; bedding and mattresses were stacked high.
Sixteen members of the Zani clan were squeezed into a classroom with several other families. They had fled their homes in northern Gaza, where Israeli troops were conducting intensive searches for Palestinian militants' tunnels, meant to funnel assailants into Israel, and rocket-launching sites.
In previous years, the children would have been up early because of the excitement of the holiday.
"Today we woke up at 6 a.m., not because the kids were playing, but because we heard shelling," said Aimani Fora, a young mother. Her children, she said, asked why Eid was not being celebrated as usual with sweets, big meals and new clothing.
"I told them Eid had been postponed because there were a lot of martyrs," she said. "A lot of people here have lost family members — we can't celebrate when they are grieving."
Times staff writer Zavis reported from Gaza City and special correspondent Sobelman from Jerusalem. Staff photographer Carolyn Cole in Gaza City, staff writer Laura King in Cairo and special correspondent Maher Abukhater in Ramallah, West Bank, contributed to this report.