"History says that when you win the presidency, you get complacent".
It is not the first time Mr Trump has remarked on security in the United Kingdom capital.
The president is addressing the NRA gathering almost three months after a deadly shooting at high school in Parkland, Florida.
The president was addressing the NRA gathering almost three months after a deadly shooting at a high school in Parkland, Florida.
Speaking at the NRA convention in Dallas Texas Friday night US President Donald Trump argued against the Iran deal made by the previous administration by arguing that the deal should not have been made as long as Iranians are shouting "Death to America".
President Trump has reiterated his support for arming highly trained teachers with weapons, in an effort to protect students.
Trump says the National Rifle Association is a "great organization that loves this country".
Under siege for the remarks that come less than three months after Parkland from left-wing activists, Trump's speech is expected to draw thousands of protesters, including actress Alyssa Milano.
He said: "The terrorists would have fled or been shot and it would have been a whole different story".
Mr Trump is due to visit the United Kingdom this summer, after previously cancelling a trip amid calls for mass protests.
NASA's Lockheed-built lander poised for long-awaited trip to Mars
The planet's thin atmosphere makes landing a challenge, and its extreme temperature swings make it hard to operate on the surface. NASA's InSight spacecraft is about to blast off for Mars - and there are 3 ETFs that can help you invest in space travel.
This is Mr Trump's fourth address to the NRA and his second in office - the first president to do so in 30 years.
"The quickest way to stop a bad guy with a gun is a good guy with a gun", Pence said Friday, echoing a favorite NRA slogan.
Back in February, Trump had praised members of the gun lobby as "great patriots" but declared "that doesn't mean we have to agree on everything".
Dr Martin Griffiths, who works at the Royal London Hospital, blamed knife and gun violence as he said: "We're doing major life-saving cases on a daily basis".
An estimated 80,000 people have attended this year's NRA convention in Dallas.
Pro-gun demonstrators and gun control advocates, many of them galvanized by the school shooting in Parkland, Florida, staged events outside the Kay Bailey Hutchison Convention Center.
Student survivors of the February 14 shooting at Marjory Stoneman Douglas High School that left 17 people dead are now leading a massive national gun control movement.
"The world is watching and we're going to come up with a solution", Trump assured a group of people affected by the nation's deadliest school shootings during an emotional White House listening session one week after the attack. He later backpedaled on that tough talk.
After meeting with families of some of the victims, Trump flirted with gun control measures the NRA opposes, such as raising the minimum age for long gun purchasers to 21.
Instead, he focused on the bill he signed into law which included $2 million in school safety funding and improvements in reporting to the background check system - all measures supported by the NRA.