A vigil held in one of London's gay districts, Old Compton Street in Soho, has featured a two-minute silence that ended with applause for the Orlando victims.
In November, the BBC took a look into what steps the US takes when admitting Syrian refugees to the country.
The New York businessman has now moved to attacking Hillary Clinton by criticising her statements after the San Bernadino attacks in California.
"I will always support the second amendment," he says.
Trump also claims Obama "is not allowing" law enforcement to do their job and suggests the US needs an "intelligence operation second to none".
In a speech in New Hampshire, Republican Donald Trump says the US must respond to "force, purpose and determination" and adds: "If we don't get tough, we're not going to have a country any more."
Mr Trump has also reiterated his call for a ban on all Muslim immigration, referencing Mateen's father's immigration as the "only reason this killer was in the country".
Vigils are being held across the UK in memory of the 49 people killed at a gay nightclub, including this one in Soho in London.
Makeshift memorials have also cropped up near US embassies in other countries.
In her first speech since the Orlando shootings, Democratic presidential candidate Hillary Clinton says the country faces a "twisted ideology and a poisoned psychology" that inspires "lone-wolf" attacks.
Vowing to make stopping these kind of solo attacks a priority of her presidency, she added: "If you are too dangerous to get on a plane, you are too dangerous to buy a gun in America."
The FBI director James Comey refuses to use the name of the man who killed 49 people at a gay nightclub in Orlando.
- Copyright: BBC
As the investigation continues, police have begun sharing details about what happened during the standoff with officers. Read more about what we know about how the attack unfolded.
- Copyright: BBC
BBC's Rajini Vaidyanathan is live on Facebook near a memorial in Orlando where she is taking your questions and reading tributes to the victims.
Former American Idol singer Clay Aiken, also a Democratic politician, says the Orlando shooting victims should be honoured by improved gay rights in the US.
Gay men and women were persecuted before Sunday, he told the BBC. For example, he said, they can still lose their jobs for being gay.
FBI director James Comey said he was "highly confident this killer was radicalised" and partially through the internet.
More from Comey:
- Mateen questioned in 2013 because he made "inflammatory and contradictory" statements, including claiming connections to both al-Qaeda and Hezbollah - two groups diametrically opposed to each other
- he told the FBI he had made the comments in anger because he thought colleagues were discriminatory and mocking him
- the FBI followed him and investigated further but closed the case after 10 months
- second investigation began because Mateen once attended the same mosque as a suicide bomber, and a person the FBI interviewed had once been concerned that he had been radicalised, but was no longer worried because he had been recently married and had a child.
The BBC has updated our page with more names of victims of the Orlando shootings, as well as details about some of them. We will continue to update as we learn about those killed.Copyright: JK Rowling/Twitter