The murder of Joe Reilly in the Poleglass area of west Belfast was "a cold-blooded and summary execution of a man in his own house," police have said.
Mr Reilly (43), is believed to have been shot a number of times by gunmen at his home in Glenwood Court.
The incident happened at about 8.15pm on Thursday.
Officers from the PSNI's Serious Crime Branch have launched a murder investigation.
Detective Chief Inspector Peter Montgomery has urged anyone in the area at the time or who has any information about the shooting to contact police.
He said: "This was a cold-blooded, summary execution of a man in his own house.
"At about eight o’clock last night a number of men, one of whom was armed with a handgun, opened the unlocked door of number 1 Glenwood Court in Poleglass.
"Two people in the house were subsequently ordered into the kitchen and made to lie on the floor. The man who lived in the house, 43 year old Joe Reilly, was kept in the living room. Mr Reilly was shot a number of times in the chest and, despite the best efforts of paramedics, he died at the scene.
"The gunmen ran out of the house by the front door and down the street. They are described as being about six ft tall, wearing dark coats and dark trousers. One was wearing dark rimmed glasses and a Celtic scarf.
"It is really difficult to know how to describe what happened last night. The adjectives ‘brutal, senseless and barbaric’ are inadequate. This was a cold-blooded summary execution of a man in his own home.
"At this early stage in the investigation, we have no motive for Joe Reilly’s murder. We will keep a completely open mind as to why he was killed."
"I have two appeals - firstly, I understand three children were seen in the area as the gunmen ran away.
"I need to talk to these children and find out what they saw.
"Secondly I appeal for anyone who was in Glenwood Court and Glenwood Crescent before or after 8pm to come forward."
At the scene on the edge of a densely-populated housing estate, an extensive cordon was put in place as Police Service of Northern Ireland (PSNI) detectives conducted their investigations.
As parents walked past the red and white police tape to drop their children at a school, just a few hundred metres away, there was a palpable sense of shock, although few people wanted to speak publicly.
One woman, who was looking on at the police line while clutching her young daughter, said the community was stunned.
It was the second shooting in the area in less than a week. On Sunday a man in his 50s was hit up to six times in the lower legs.
Justice Minister Claire Sugden described the shooting as an attack on the whole community.
The independent MLA said: "I unreservedly condemn this appalling act and I offer my deepest sympathies to the family.
"Murder is an attack against our entire community and those responsible have no place in society while they continue to be involved in crime."
SDLP West Belfast MLA Alex Attwood condemned those behind the attack.
Mr Attwood said: "I completely condemn the latest barbaric attack on a man in West Belfast. These savage assault had no place in our past and they have no place in our society today.
"This is a very small group of people, unrepresentative of the local community, who have now repeatedly taken the law into their own hands.
"The best response that we can make is to provide information to law enforcement and ensure that these individuals are caught, arrested and prosecuted under the full weight of the law."
In a joint statement, the First and Deputy First Ministers Arlene Foster and Martin McGuinness united in condemnation of the gun attack.
They said: "We unreservedly condemn the murder of a man shot dead in his home in Poleglass last night.
"This savage killing is senseless and has achieved nothing. The perpetrators care only for themselves."
It is understood an armed gang forced their way into the house at Glenwood Court shortly after 8pm and shot Mr Reilly in the chest.
He was taken to hospital and later declared dead.
Northern Ireland Secretary James Brokenshire has also expressed outrage.
He wrote on Twitter: "Appalled by senseless and brutal murder in west Belfast last night. My thoughts are with family & everyone affected by this."
West Belfast Policing Chief Inspector Norman Haslett expressed sympathy to the Reilly family and said officers were determined to provide a comprehensive service to keep local people safe.
He said: "I would like on behalf of the Police Service of Northern Ireland to express my sincere condolences to the family of Joe Reilly following the terrible events in West Belfast yesterday evening. This was a brutal, senseless and horrendous killing of a man in cold blood and I condemn it in the strongest possible terms. There is no place for such activity in our society or in the community of west Belfast.
"I want to take this opportunity to reassure the community ofwest Belfast that police have, and will continue to deploy our resources in a significant and meaningful manner to prevent harm and to protect the community.
"Such harm comes in various forms and on a daily and nightly basis, around the clock, police will be on the streets of west Belfast to continue to take drugs from our streets, to stop child sexual exploitation, to address anti-social behaviour and death driving and to prevent and detect burglaries. The local community and the criminals can continue to expect to see visible policing in the coming weeks and months.
“Police in west Belfast are providing a much needed and increasingly valued service and we remain determined to work together with the community to keep people safe. The vast majority of the community of West Belfast are law abiding people who just want to live in peace. Those people who did what they did last night want to try and drag us back to the past are not supported by the community.
“I would like to reiterate our appeal for anyone who saw or heard anything yesterday evening to come forward and give us the information which we need to bring those responsible to justice. There may be people out there who have never picked the phone up to talk to us in the past. If you know anything at all, think about what has happened to Joe Reilly, think about what his family are now going through and please make the phone call and help us catch those who did it.”