America is absorbing the shock of another bloody mass shooting, a day after an 18-year-old man wearing body armour and carrying assault rifles entered an elementary school in Texas and gunned to death at least 19 children and two adults.

The attack on Robb elementary school in Uvalde, 85 miles west of San Antonio, was the deadliest gun rampage in an American school in almost a decade. It prompted passionate calls for tougher gun controls led by Joe Biden but matched by equally stringent demands for more armed guards in schools from the gun lobby and Republicans.

The shooting began to unfold at 11.32am on Tuesday when the shooter, who is believed to have posted photographs of what he called “my guns” on Instagram four days previously, opened fire in a classroom of nine- and 10-year-olds. He carried an assault-style weapon and wore a tactical vest in which he is believed to have held large quantities of ammunition.

Chris Olivarez of the Texas department of public safety gave CNN chilling details of what happened. He said the shooter barricaded himself into a classroom where he opened fire on children and two teachers.

All the victims were reported to be from the same classroom, Olivarez said. A Swat team eventually broke into the room, shooting the gunman dead.

More than 200 rounds of ammunition were discovered with the shooter’s body, in 30-round magazines. It emerged on Wednesday that the gunman obtained his weapons legally over a three-day period this month, shortly after his 18th birthday.

He bought two semi-automatic AR-15 style rifles at a federally registered gun dealership on 17 and 20 May, and 375 rounds of ammunition on 18 May.

Earlier, the shooter shot his grandmother at her home in Uvalde. She was in critical condition. The Texas governor, Greg Abbott, said at a press conference on Wednesday that she had called the police before being shot.

Abbott said the shooter posted on Facebook three times before his attack. In the first, posted 30 minutes before going to the school, he said he would shoot his grandmother. The second said, “I shot my grandmother.” And the third, posted about 15 minutes before the attack, said: “I’m going to shoot an elementary school.”

Abbott also said the shooter’s grandmother had called the police before he shot her.

Uvalde is a small town of about 16,000, overwhelmingly Hispanic.

Among the confirmed victims were two adults: Eva Mireles, 44, a bilingual special education teacher who was reportedly killed as she tried to shield her pupils, and the co-teacher Irma Garcia, who had taught at the school for 23 years and had four children of her own. Among the children publicly identified, the youngest was eight. Several were 10.

They included eight-year-old Uziyah Garcia, described by his grandfather as “the sweetest little boy that I’ve ever known”; Xavier Lopez, 10, who his cousin said was “very bubbly, loved to dance”; and Amerie Jo Garza, who celebrated her 10th birthday two weeks ago.

The shooting left more people dead than any US school shooting since Sandy Hook elementary school in Connecticut in December 2012. The impact was compounded by its timing, just 10 days after another 18-year-old gunman opened fire on grocery shoppers, most of them Black, in a supermarket in Buffalo, New York, killing 10.

On Tuesday night, a visibly shaken Biden urged Americans to resist the powerful gun lobby, which he blamed for blocking tougher firearms laws. Flags will be flown at half-mast until sunset on Saturday.

“As a nation, we have to ask, ‘When in God’s name are we going to stand up to the gun lobby?’” Biden said. “When in God’s name are we going to do what has to be done? Why are we willing to live with this carnage?” He was “sick and tired of it”, he said, adding: “We have to act.”

But Republican leaders – not least in Texas itself – were just as robust in their calls for more guns in schools. Ted Cruz, the US senator, said just a few hours after the attack the best way to keep kids safe was to have armed officers on campus.

Ken Paxton, Texas’s attorney general, told the rightwing news outlet Newsmax the way to save lives was to have “teachers and other administrators who have gone through training and who are armed”.

Their arguments were belied, however, by the facts of the Uvalde massacre. As the shooter entered the school, two local officers and a school guard opened fire but failed to stop him.

Texas has led the US with a steady stream of initiatives loosening restrictions on firearms ownership. Last year its Republican governor, Abbott, enacted a law that removed almost all restraints on carrying handguns in public – despite the fact Texas has been the scene of several of the most horrifying mass shootings in US history.

On Wednesday Abbott said other than the shooter posting on Facebook three times roughly 30 minutes before his attack, “there was no meaningful forewarning of this crime”. He said he asked the sheriff and mayor, “What is the problem here? And they were straightforward and emphatic. They said we have a mental we have a problem with mental health illness in this community.”

As Abbott’s press conference concluded, Beto O’Rourke approached the stage and told Abbott “you are doing nothing”. O’Rourke, the former presidential candidate who is running against Abbott in November for the gubernatorial seat, could be heard telling Abbott that the shooting was “predictable” because of his inaction and mentioned the 2019 shooting in El Paso where 23 people were killed in a Walmart store.

“This is on you until you choose to do something,” O’Rourke said.

“Sir, you are out of line,” the mayor of Uvalde, Don McLaughlin, shouted at O’Rourke in an attempt to get him to leave the auditorium. Last year McLaughlin backed Abbott’s even more rightwing Republican opponent for governor.

Speaking on Wednesday, Biden said he and first lady Jill Biden would travel to Uvalde “in the coming days” to offer what comfort they can to “a community in shock and grief and trauma.

The president and other campaigners for greater gun control face the numbing reality that in the US there are more firearms in circulation than there are people. The pandemic has seen a dramatic uptick in gun sales, and with it a surge in gun deaths.

In the last decade there have been at least 3,500 mass shootings, defined as incidents killing or injuring four or more people, according to the Gun Violence Archive. The rate of deaths of children under 14 has also risen sharply since the pandemic.

There were heartbreaking scenes outside the Uvalde school. Hours after the attack, distraught families were still awaiting word on whether their children had survived, the silence broken repeatedly by screams and wailing.

“My heart is broken today,” said Hal Harrell, the school district superintendent. “We’re a small community, and we’re going to need your prayers to get through this.”

The school was preparing for its final day on Thursday. Themed days had been organised, with children asked to come on Tuesday dressed as “Footloose and Fancy”.

Adolfo Cruz, 69, said he drove to the school after receiving a terrifying call from his daughter. He was waiting for news of his 10-year-old great-granddaughter, Eliajha Cruz Torres, and it was the heaviest moment of his life, he said.

In strong international reactions to the shooting, Pope Francis said he was “heartbroken”, adding: “It is time to say ‘enough’ to the indiscriminate trafficking of weapons.” Emmanuel Macron said the French people shared Americans’ shock and grief at the “cowardly” shooting.

Ukraine’s president, Volodymyr Zelenskiy, said he was “deeply saddened by the news of the murder of innocent children”.