Israeli airstrikes flattened mosques over the heads of worshipers. At least two hospitals, and two centers run by the Palestine Red Crescent Society, have been hit. So have two schools run by the U.N. agency that helps Palestinian refugees.

Israel’s fighter jets and artillery have struck targets in Gaza frequently over the years as part of the longstanding conflict between the Palestinians and Israel. But from the first day of this new war, residents of Gaza and health authorities have said that this time the strikes have been indiscriminately hitting structures that haven’t regularly been targeted in the past, such as schools, hospitals and mosques. Few warnings have been given before strikes, Gazans say, and entire families have been killed in their homes, according to the Gazan Health Ministry.

The strikes are coming as part of Israel’s response to the attack on Saturday, when hundreds of Palestinian gunmen swept across Israel’s border with Gaza, killing at least 1,000 people, and taking around 150 hostages, including children and the elderly. On Monday, Israel’s defense minister announced a “complete siege” of Gaza, saying “no electricity, no food, no water, no fuel” would be allowed in.

Israel has said its strikes are targeting all sites connected with Hamas, the armed Palestinian group that controls the Gaza Strip, including the homes of members. Israel said it believes Hamas members are hiding in homes, schools and hospitals. Hamas members are Palestinians from Gaza, so they live among the community.

But the Israeli strikes have shocked Palestinians. Israel has given general warnings for people to leave certain neighborhoods or towns, but has acknowledged they are not as extensive or specific as they have been in the past. Residents said there have been few specific warnings and that they have nowhere to go anyway.

“What they are doing shouldn’t be allowed,” said one woman, 25, who fled to the Al Shifa hospital cradling her week-old baby after an airstrike hit near their home in northern Gaza, close to the border with Israel. She did not give her full name.

For three days, she and 19 members of her family had been sheltering in a hall in a part of the hospital that was under construction. They were joined by hundreds of others who fled the strikes and slept in the hallways or in the courtyards outside.

On Tuesday, Israeli warplanes continued to pound Gaza with airstrikes, reducing some buildings to rubble. Palestinian authorities said 900 people had been killed and about 168 buildings had been damaged since Saturday, when the strikes began, among them seven hospitals and 48 schools. At least 4,500 Palestinians have been injured, according to Gaza’s health ministry. It was not clear how many of the casualties were from the Palestinian fighters who carried out Saturday’s attack.

Volker Türk, the United Nations high commissioner for human rights, on Tuesday warned that a full siege of Gaza would exacerbate the “already dire” humanitarian situation in the coastal territory and would hurt hospitals’ ability to treat the growing number of wounded.

Gaza’s medical system and infrastructure have come under attack by Israeli airstrikes, the health ministry in the territory said on Monday, as hundreds of wounded fill hospital operating rooms and intensive care units. At least five medical workers have been killed, the ministry said. .

The heavy Israeli bombardment has made movement in the streets dangerous and ambulances have had difficulty transporting the dead and wounded. Instead, people have at times relied on borrowed vehicles, tuk-tuks or motorcycles.

The health ministry said that at least nine ambulances had been struck since Saturday.

Gaza, a small, densely populated enclave that is home to more than two million people, has been under a severe blockade imposed by Israel and Egypt for 16 years, limiting what can go in, including medicines and medical equipment.

“The hospitals in Gaza are in a very critical situation as a result of this oppressive siege, and that has led to a big shortage of medicine and medical tools and fuel,” Mr. al-Qidra said, referring to the new bombardment. “Everyone needs to bear responsibility to save the medical work in Gaza.”

Three days into the war with Gaza, Israeli military commanders began to speak about a “change of paradigm” in its airstrikes on Gaza.

“This is unlike anything we have had in the past, and we need to use different language and different terminology regarding our assault activities in Gaza,” Daniel Hagari, an Israeli military spokesman, said in a briefing on Tuesday. “This is not like previous rounds.”

Telephone and internet service were cut off in many parts of Gaza on Monday after an Israeli strike hit the building housing the Palestine Telecommunications Company in the city center. The U.N. humanitarian agency said Israeli airstrikes have damaged water, sanitation and hygiene facilitates affecting more than 400,000 people in Gaza.

And after days of strikes, entire neighborhoods no longer look like they did just a couple of days ago.

In the upscale Gaza City neighborhood of Al-Rimal, where the Israeli army said on Tuesday that it had carried out its main airstrikes overnight, buildings were so damaged they bled into one another.

Thousands of people fled Al-Rimal, but many have nowhere to go; Gaza has no bomb shelters and those who went to the homes of relatives often found that they too were fleeing.Another woman, 38, was at the hospital morgue on Tuesday, waiting along with other family members to take the bodies of her niece and her two young daughters so that they could be buried. On Monday, she said, the three were killed when an airstrike hit their home and they were crushed under the rubble.

“No warning,” she said, her eyes bloodshot and swollen from crying. “If they had called them, they would have left the house.” She did not give her full name.

Lt. Col. Richard Hecht of the Israeli Defense Forces said that the Israeli Air Force was too stretched to fire the warning strikes — known as “roof knocks” — that it has fired in previous Gaza conflicts to encourage Palestinian civilians to leave an area before it is hit with larger missiles. He said that Israel was telling Gazans to move from areas that would be targeted, and advised them to leave through the Rafah border crossing with Egypt.

But hours later, on Tuesday, the Israeli military bombed the crossing, shutting it down.

“We are asking for someone to stand with us; we are asking at least warn us before and then strike so we can ensure we get out safely,” the 25-year-old woman said.

Her family had to wait for an ambulance to come and take them south to Gaza City. But even far from the border, the Israeli strikes continued to hit nearby.

“From the fear, the children can’t sleep,” she said. “We try to calm them. We tell them, ‘Don’t be afraid, God is with us.’”