At least 50 people have been killed and 53 injured in a "terrorist" attack on a gay nightclub in Orlando, the deadliest mass shooting in US history, the city's Mayor has said.

The attack was carried out at Pulse nightclub in the early hours of Sunday morning. Florida authorities have declared a state of emergency.

Buddy Dyer, Orlando's Mayor, announced the casualties – higher than the 20 deaths originally reported – at a press conference this morning.

Mr Dyer said in a statement: "We are dealing with something we never imagined.

"Because of the scale I have called Governor Scott to request a state of emergency and we are issuing a state of emergency in the City."

Police identified the attacker as Omar Mateen, a 29-year-old from Port St. Lucie, Florida – more than 100 miles away from Pulse. His family is from Afghanistan, while Mr Mateen is believed to have been born in the United States.

Police said the shooting was a “terror incident” and said the shooter was “organised and well-prepared.” They said he had an assault rifle and handgun, and was not from Orlando.

Mr Mateen, who is said by the FBI to have "leanings" towards extremism and Islamic State, was involved with a stand-off with officers for around three hours before SWAT teams stormed the building and shot him dead.

Police chief John Mina also said the shooter had a “suspicious device”.

Mr Mina said the focus was now on securing the night club and the surrounding area, including the suspect’s vehicle, a van parked outside the club. Once these areas were secure, they would be able to start removing bodies and notifying family members.

Mr Mina said: “I want to commend the heroic and courageous actions of the first responders who were involved in an exchange of gunfire and the Swat team who were able to save up to 30 potential victims.”

Michael Cheatham, a surgeon with Orlando Health, said local hospitals had implemented a “mass casualty plan” after the shooting and spent the morning operating on a number of victims, “many of them critically ill as a result of their injuries”.

Mr Cheatham asked people to consider donating blood as a way of assisting in the wake of the attack.

He said: “Blood is a wonderful gift. That can be arranged through local blood banks, don’t come to the local hospitals.”

OneBlood, an organisation that promotes and facilitates blood donation, announced there was an urgent need for O Negative, O Postive and AB Plasma blood donors in the wake of the attack.

In pictures: Orlando nightclub shooting Show all 30 1 /30 In pictures: Orlando nightclub shooting In pictures: Orlando nightclub shooting Friends and family grieve after a list of hospitalised victims was released, implying the death of those who weren't on the list and hadn't been heard from, outside a Hampton Inn & Suites hotel near the Orlando Regional Medical Center AP In pictures: Orlando nightclub shooting People mourning for victims of the mass shooting near the Pulse gay nightclub in Orlando AFP/Getty Images In pictures: Orlando nightclub shooting Mina Justice speaks to a reporter discussing texting with her son Eddie who was in a bathroom at Club Pulse in Orlando. It has now been confirmed that Eddie Justice was among the 50 people killed in the massacre AP In pictures: Orlando nightclub shooting FBI agents investigate near the damaged rear wall of the Pulse Nightclub Getty Images In pictures: Orlando nightclub shooting Terry DeCarlo, executive director of the LGBT Center of Central Florida (C) is comforted by Orlando City Commissioner Patty Sheehan (R) after a shooting involving multiple fatalities at a nightclub in Orlando AP In pictures: Orlando nightclub shooting FBI agents investigate the damaged rear wall of the Pulse Nightclub Getty Images In pictures: Orlando nightclub shooting Police officials investigate the back of the Pulse nightclub after a shooting involving multiple fatalities at the nightclub in Orlando AP In pictures: Orlando nightclub shooting A member of the Medical Examiners office wheels a body to a vehicle from the mass shooting at Pulse Nightclub Getty Images In pictures: Orlando nightclub shooting A photograph posted by the Orlando Police Department on Twitter with the words, "Pulse shooting: In hail of gunfire in which suspect was killed, OPD officer was hit. Kevlar helmet saved his life", in reference to the operation against a gun man inside Pulse night club in Orlando REUTERS In pictures: Orlando nightclub shooting FBI, Orlando Police Department and the Orange County Sheriff's Office personnel investigate the attack at the Pulse nightclub in Orlando AP In pictures: Orlando nightclub shooting Annette Stubbs, a pastor at a local church, prays for victims a few blocks from a crime scene at the nightclub where a mass shooting took place AP In pictures: Orlando nightclub shooting Aimee McCarthy from Jacksonville, gives blood at the oneblood facility, to help the victims from a mass shooting at a nightclub in Orlando AP In pictures: Orlando nightclub shooting Police forensics investigators work at the crime scene of a mass shooting at the Pulse gay night club in Orlando REUTERS In pictures: Orlando nightclub shooting An injured person is escorted out of the Pulse nightclub after a shooting rampage AP In pictures: Orlando nightclub shooting An injured man is escorted out of the Pulse nightclub after a shooting rampage AP In pictures: Orlando nightclub shooting An injured man is escorted out of the Pulse nightclub after a shooting rampage AP In pictures: Orlando nightclub shooting Law enforcement officials work at the Pulse nightclub following a fatal shooting AP In pictures: Orlando nightclub shooting Orlando Police officers direct family members away from the nightclub AP In pictures: Orlando nightclub shooting The scene outside the Orlando gay club where multiple people have been shot AFP/Getty Images In pictures: Orlando nightclub shooting The injured are treated in the street outside Pulse in Orlando following the shooting EPA/Univision In pictures: Orlando nightclub shooting Two witnesses, Jermain Towns (left) and Brandon Shuford, wait down the street for news following shooting and hostage stand-off at the Pulse nightclub. Mr Towns said his brother was in the club at the time AP In pictures: Orlando nightclub shooting Emergency services at the scene. Ambulance crews and firefighters were outside the club alongside police. EPA In pictures: Orlando nightclub shooting People treating the wounded on the street EPA In pictures: Orlando nightclub shooting Friends and family members embrace outside the Orlando Police Headquarters during the investigation of a shooting at the Pulse night club Reuters In pictures: Orlando nightclub shooting Law enforcement agencies and local city representatives give a news conference in the wake of a mass-casualty shooting at the Pulse nightclub in Orlando Rex In pictures: Orlando nightclub shooting Orlando police officers seen outside of Pulse nightclub after a fatal shooting and hostage situation in Orlando, Florida Getty In pictures: Orlando nightclub shooting Friends and family console one another outside the Orlando Police Headquarters In pictures: Orlando nightclub shooting U.S. President Barack Obama speaks about the worst mass shooting in U.S. history that took place in Orlando REUTERS In pictures: Orlando nightclub shooting Imam Syed Shafeeq Rahman, the Imam of the mosqe that mass shooter Omar Mateen attended, speaks to the media in Fort Pierce. The imam said that the suspect never gave any indication he was capable of such violence.Omar Mateen attended evening prayers three or four times a week at the Islamic Center of Fort Pierce, bringing his son who is about four or five years old AFP/Getty Images In pictures: Orlando nightclub shooting Omar Mateen has killed 50 people at a gay nightclub after pledging allegiance to Isis in the deadliest mass shooting in US history Myspace

The Florida chapter of the Council on American-Islamic Relations (CAIR-FL) is calling on the Muslim community to take part in a blood donation drive for those injured in the attack.

President Barack Obama ordered the federal government to provide any assistance necessary to local law enforcement officials investigating a shooting.