Presidential candidate Donald Trump has called on President Barack Obama to take a stand against “radical Islamic terrorism” or else “resign in disgrace” after a mass shooting in a Florida nightclub claimed 50 lives.

Reacting to gunman Omar Mateen’s rampage through Orlando’s Pulse gay club, Trump called America’s leadership “weak and ineffective” and sought credit for being “right on radical Islamic terrorism”.

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“Appreciate the congrats for being right on radical Islamic terrorism, I don’t want congrats, I want toughness & vigilance. We must be smart!” Trump tweeted to his nearly nine million followers.

“Is President Obama going to finally mention the words radical Islamic terrorism? If he doesn’t he should immediately resign in disgrace!

“What has happened in Orlando is just the beginning. Our leadership is weak and ineffective. I called it and asked for the ban. Must be tough.”

Is President Obama going to finally mention the words radical Islamic terrorism? If he doesn’t he should immediately resign in disgrace!

— Donald J. Trump (@realDonaldTrump) June 12, 2016

Mateen, from Port St. Lucie in Florida, was killed by police who used an explosive device to distract him and rescue around 30 people he had taken hostage.

Mateen had called 911 to swear allegiance to the leader of Islamic State shortly before the massacre, according to law enforcement officials.

In a speech hours after the attack, Obama condemned the attack – the worst of its kind in US history – as an “act of terror and hate” in a place of “solidarity and empowerment” for gays and lesbians.

Watch President Obama’s speech below.

Democratic candidate Hillary Clinton also took to Twitter to comment on the attacks, emphasising the importance of gun control.

“To the LGBT community: please know that you have millions of allies across our country. I am one of them,” Clinton wrote.

“We need to keep guns like the ones used last night out of the hands of terrorists or other violent criminals.”

“This is a time to stand together and resolve to do everything we can to defend our communities and country.” —Hillary on the attack in FL

— Hillary Clinton (@HillaryClinton) June 12, 2016

The hashtag #LoveWins, which went viral last year after the same-sex marriage ruling by the U.S. Supreme Court, has been adopted by the world to express support and grief after the shooting.

Sympathy and grief poured in from around the world, from London mayor Sadiq Khan to Australian celebrity Ruby Rose, to express sadness at the death of more than 50 people in the mass shooting in Florida.

I am gay and I am not afraid. Love conquers all. #PrayforOrlando

— Ricky Martin (@ricky_martin) June 12, 2016

I played Orlando last week. Friday. I played LA, after pride, last night. All I see is LOVE, DANCING, FREEDOM, HAPPINESS from the Dj booth.

— Ruby Rose (@RubyRose) June 12, 2016

Sympathy not enough @TurnbullMalcolm Send a real message. U talk freedom Give us our rights. #marriageequality #standinsolidarity #lovewins

— Magda Szubanski (@MagdaSzubanski) June 12, 2016

These 2 never let go of each other the entire 15 minutes I stood by them. #lovewins pic.twitter.com/kXOYmNGWVD

— Matthew Risch (@MatthewRisch_) June 12, 2016