HOUSTON — “The existence of evil in our world is not a reason to disarm law-abiding citizens,” President Trump told the National Rifle Association convention at Houston in the wake of the massacre at Uvalde, Texas. On the contrary, he said, “the existence of evil is one of the very best reasons to arm law-abiding citizens.” 

The former president was the top headliner at the gathering, which came just three days after the shooting at Uvalde and as the nation grappled with revelations that students trapped inside a classroom with the gunman repeatedly called 911 during the attack — one pleading “Please send the police now” — as officers waited in the hallway for more than 45 minutes.

The NRA had said that convention attendees would “reflect on” the shooting at the event and “pray for the victims, recognize our patriotic members and pledge to redouble our commitment to making our schools secure.” The meeting was the first for the troubled organization since 2019, following a two-year hiatus because of the COVID-19 pandemic.

The NRA has been trying to regroup following a period of serious legal and financial turmoil that included a failed bankruptcy effort, a class-action lawsuit and a fraud investigation by New York’s attorney general. Once among the most powerful political organizations in the country, the NRA has seen its influence wane following a significant drop in political spending.

The group’s embattled chief executive, Wayne LaPierre, opened the program with remarks bemoaning the “21 beautiful lives ruthlessly and indiscriminately extinguished by a criminal monster.” Still, he said that “restricting the fundamental human rights of law-abiding Americans to defend themselves is not the answer. It never has been.”

Later, several hundred people in the auditorium stood and bowed their heads in a moment of silence for the victims of the shooting. 

Mr. Trump accused Democrats of trying to exploit the tragedy and demonizing gun owners.

“When Joe Biden blamed the gun lobby he was talking about Americans like you,” Mr. Trump said, referring to the president’s emotional plea in a national address asking, “When in God’s name are we going to stand up to the gun lobby?”

Mr. Trump called for overhauling school security and the nation’s approach to mental health, telling the group every school building should have a single point of entry, strong exterior fencing, metal detectors and hardened classroom doors and every school should have a police officer or armed guard on duty at all times. He also called yet again for trained teachers to be able to carry concealed weapons in the classroom.

He and other speakers overlooked the security upgrades that were already in place at the elementary school and did not stop the gunman, who entered the building through a back door that had been propped open.

According to a district safety plan, Uvalde schools have a wide range of safety measures in place. The district had four police officers and four support counselors, according to the plan, which appears to be dated from the 2019-20 school year. It also had software to monitor social media for threats and software to screen school visitors.

Security experts say the Uvalde case illustrates how fortifying schools can backfire. A lock on the classroom door, for instance — one of the most basic and widely recommended school safety measures — kept victims in and police out.

Senator Cruz, who, like Mr. Trump, is considered a potential presidential candidate in 2024, railed against Democrats’ calls for universal background checks for gun purchases and bans of assault-style weapons and instead pointed to broken families, declining church attendance, social media bullying and video games as the real problems.

“Tragedies like the event of this week are a mirror forcing us to ask hard questions, demanding that we see where our culture is failing,” he said. “We must not react to evil and tragedy by abandoning the Constitution or infringing on the rights of our law-abiding citizens.”

Governor Noem of South Dakota, another potential presidential contender, said calls to further restrict gun access are “all about control and it is garbage. I’m not buying it for a second and you shouldn’t, either.”

Some scheduled speakers and performers backed out of the event, including several Texas lawmakers and “American Pie” singer Don McLean, who said “it would be disrespectful” to go ahead with his act after the country’s latest mass shooting. The lieutenant governor of Texas,  Dan Patrick, said Friday morning that he had decided not to speak at an event breakfast after “prayerful consideration and discussion with NRA officials.”

“While a strong supporter of the Second Amendment and an NRA member, I would not want my appearance today to bring any additional pain or grief to the families and all those suffering in Uvalde,” he wrote in a statement.

Governor Abbott, who was to attend, addressed the convention by prerecorded video instead.

Outside the convention hall, protesters gathered in a park where police set up metal barriers — some holding crosses with photos of the Uvalde shooting victims.

“Murderers!” some yelled in Spanish. “Shame on you!” others shouted at attendees.

Among the protesters was singer Little Joe, of the popular Tejano band Little Joe y La Familia, who said in the more than 60 years he’s spent touring the world, no other country he’s been to has faced as many mass shootings as America.

“Of course, this is the best country in the world,” he said. “But what good does it do us if we can’t protect lives, especially of our children?”

A former Democratic congressman, Beto O’Rourke, who is challenging Mr. Abbott in the governor’s race, ticked off a list of previous school shootings and called on those attending the convention to “join us to make sure that this no longer happens in this country.”

While President Biden and Democrats in Congress have renewed calls for stricter gun laws after the Uvalde shooting, NRA board members and others attending the conference dismissed talk of banning or limiting access to firearms.

A maintenance worker for Southwest Airlines in Houston who was attending the NRA meeting, Samuel Thornburg, 43, said: “Guns are not evil. It’s the people that are committing the crime that are evil. Our schools need to be more locked. There need to be more guards.”

There is precedent for the NRA to gather during local mourning and controversy. The organization went ahead with a shortened version of its 1999 meeting in Denver roughly a week after the deadly shooting at Columbine High School in Colorado.

Though personal firearms are allowed at the convention, guns were not permitted during the session featuring Trump because of Secret Service security protocols.