The message popped up at 10:55 a.m. on Thursday: “Looking for volunteers to help unload a truck of equipment for soldiers. We are at the Museum of Tolerance. Come, there is nobody here.”

Barely four minutes after the note appeared in the 1,000-member WhatsApp group for volunteers in the Jerusalem area, Hadas Duchan showed up and got to work dismantling crates of thermal underwear, fleece jackets, hats and other gear procured by an American nonprofit organization.

“It helps you get over your feelings of helplessness,” said Ms. Duchan, 34, an artist whose two brothers are among the 360,000 reservists recently mobilized by Israel’s military after the Hamas terrorist attacks Saturday that left at least 1,200 dead.

As Israel signals that a ground invasion of Gaza could come any day, its shocked and traumatized citizens are pouring their anxious energy into raising funds and collecting goods for those in need: soldiers; survivors of the atrocities; hospitals treating the thousands of wounded; and people whose loved ones were killed, are still missing or are being held hostage in Gaza.

Thousands of grass-roots initiatives have also sprung up across the country, many organized on social media.

Mothers were donating breast milk for orphaned infants. Dozens of florists and events designers are making hundreds of funeral wreaths and bouquets for hospitals instead of bridal bouquets and table arrangements, working out of a refrigerated agricultural warehouse at a cooperative near Israel’s international airport.

Some large nurseries have been donating the flowers, which were coming in by the truckload, said Elki Jacobs, a designer.

“It’s extraordinary,” she said. “Right now, the spirit of Israel is here in the streets, in the restaurants, packing clothes.”

Ms. Duchan said she had joined a number of volunteer-oriented WhatsApp groups on Saturday night, as the scale of the horrors from the Hamas assault was just beginning to become clear. She spent Wednesday distributing food and medicine to families in Ofakim, a desert city about 15 miles from Israel’s border with Gaza that was infiltrated by the gunmen.

“My heart aches also for the civilians in Gaza who were not to blame for this,” Ms. Duchan said. But she added that, for many Israelis, the outpouring of grief and help was personal.

“I don’t know anyone who doesn’t know someone who was there and who was hurt or killed or kidnapped,” she said of the attacks in the Israeli villages near Gaza’s border and at the musical festival that ultimately became a killing field.

Just around the corner from where she was working at the Museum of Tolerance on Thursday, staff members of the gourmet kosher restaurant 1868 were preparing 600 meals: schnitzel, kebabs and a vegetarian option. The chef, Yankale Turjeman, said that customers were donating money for the food, which would be sent to hospitals, army bases and Sderot, a city near the border with Gaza that suffered a significant number of casualties in the Hamas assault.

With many weddings canceled, a Jerusalem wedding hall was turning out thousands of meals a day for soldiers.

Similar acts of support were cropping up all over Israel.

For months before the tragedy, a deeply divided Israel had been seething with large street protests held by opponents of the right-wing government’s plan to weaken the judiciary.

Members of Israel’s military reserves were at the vanguard of the protest movement, and thousands of them had threatened to quit volunteering for service, saying that the government had broken its contract with them by undermining the democratic system they had signed up to fight for.

Many Israelis are now angry at the government led by Prime Minister Benjamin Netanyahu for being caught so off-guard by, and slow to respond to, the Hamas incursion.

But Brothers and Sisters in Arms, an Israeli organization that had been spearheading the reservists’ antigovernment protest, abruptly switched gears as details of the bloodshed emerged over the weekend, and urged all reserve volunteers to report for duty.

The protest groups have also repurposed their organizational and logistical structure to provide civilian assistance. Brothers and Sisters in Arms is heading up a civilian operations room with hundreds of volunteers at an exhibition center in Tel Aviv, collecting equipment such as head torches and mobile phone chargers for soldiers and preparing packages for families in mourning.

Israel’s high-tech community has also turned from protesting against the government to supporting the people hurting. Dozens of companies have come together to raise funds and provide assistance.

Roy Zur, the chief executive of a cybersecurity company, said that he and a few other executives got together on Saturday night to brainstorm and on Sunday met with the producer of the music festival. The producer, who lost his wife and many friends in the massacre, had thousands of messages in his phone sent by participants in the rave as they came under attack and from their relatives desperately seeking information about their whereabouts.

The tech companies took the data and worked quickly to mesh names with photos and videos using civilian facial recognition technology.

“It was probably the most thorough database at a time when everything was still chaotic,” Mr. Zur said. “We provided a lot of critical information to government officials.”

The high-tech operations room is now also collecting clothing and equipment, and one team is focusing on building cases of possible war crimes by the assailants.

On the Jerusalem WhatsApp group, a guitarist and a singer were asking where they could go to help lift the sadness. A group of psychotherapists was offering free support to survivors of the rave and other trauma victims. A call went out for people to attend prayers at a shiva, or mourning house.

Michael Kart, 45, an architect, arrived at the museum within about 20 minutes of seeing the call for help to unload the truck. But he was too late. Enough volunteers had beat him to it and the job was done. He said he would wait for the next one.

“It’s impossible to stand on the side,” he said. “It’s a small country. It’s sad that we united because of this disaster, but we are one.”