Two further fatalities were confirmed at noon this Saturday, October 8.
A further eight injured victims are being treated in hospitals.
A search operation, involving sniffer dogs, is continuing as a number of people are still missing, with many others injured.
The explosion happened at an Applegreen service station in the village of Creeslough on Friday afternoon.
The blast destroyed the building and a section of an apartment building.
Gardaí said three fatalities were confirmed on Friday and four were confirmed overnight.
Eight people were also taken to hospital.
Many emergency services vehicles remained at the scene overnight, including fire services from sides of the border, gardaí and civil defence, while a coastguard helicopter airlifted some of those who were injured in the blast from Letterkenny University Hospital to Dublin.
On Friday afternoon, the Letterkenny hospital said it was dealing with “a serious incident involving multiple injured people requiring immediate attention”.
It stood down its major emergency standby protocol at about 10 pm local time.
Other emergency services from the North of Ireland have been assisting in rescue efforts.
Creeslough is a small village about 15 miles from Letterkenny and 30 miles from the border with the North of Ireland.
In a statement on Twitter the Applegreen company said the news was “devastating”.
“Our thoughts and prayers are with the families and friends of the deceased, those who have been injured, and the wider Creeslough community,” they tweeted.
The company said that the supermarket and Applegreen outlet had been operated by local partners at “the heart of the community”.
“Like the rest of Ireland, tonight we stand in solidarity with that community.”
Taoiseach Micheál Martin expressed his sympathies to the family and friends of those who had died and to the entire community of Creeslough, on this “darkest of days for Donegal and the entire country.”
“People across this island will be numbed by the same sense of shock and utter devastation as the people of Creeslough at this tragic loss of life,” he added.
He thanked members of the emergency services, from across the north-west and Northern Ireland.
Adrian MacAuley from Derry owns an apartment behind the petrol station.
“The place seems to be devastated,” he said.
“Our thoughts are with anybody that was injured in it because Creeslough is a fantastic community,” he added.
Local resident Kieran Gallagher’s house is about 150 yards from the scene.“I was in my house at the time and heard the explosion. Instantly I knew it was something – it was like a bomb going off,” he said.
He has since been helping with the aftermath, directing traffic and helping the emergency services.
“Everybody from everywhere around Creeslough are here to help. All the businessmen and workmen are here – we have to be strong and help each other through this.
Earlier on Friday, Pearse Doherty described the incident as “the worst nightmare” for the families.
“This is a quiet, close-knit village and this is the only shop on the town. It’s the petrol station, it’s the deli counter and the Post Office is there too,” he said.
“At quarter past three a massive explosion, which could be heard for miles around ripped right through the building.
“There are people still trapped in the building and emergency services are doing everything they can,” he said.
“People are being airlifted away to our hospitals,” he added.
Donegal Fine Gael TD Joe McHugh said people in the area were “numb” with shock.
“They’re trying to find out about friends and relatives and playing a waiting game for news,” he told BBC News NI.
“It’s a very surreal situation here and we know there will be a difficult times ahead.
“The building is somewhere people meet. It is a tragedy”.
Bishop of Raphoe, Alan McGuckian, told BBC News NI everyone in the county was “shocked and shaken beyond words”.
Speaking from the scene, he said Creeslough was a “peaceful and quiet town” and it was a “terrible day”.
“There is a dark cloud over Creeslough as this scene unfolds before us – it is very sombre here and there are silent prayers being said.”
Donegal independent councillor John O’Donnell said his understanding was that the incident was caused by a gas explosion.
“A lot of people are very seriously injured – we haven’t heard numbers yet,” he told BBC Radio Ulster’s Evening Extra programme on Friday afternoon.“There are three helicopters at the scene – we have the mountain rescue helicopter, we have the RNLI and we also have the army helicopter.
“The feedback I am getting from family and friends who are unfortunately down at the scene and in the area is that the whole place is chaos at the minute. Everybody is very taken aback and devastated at what is happening.
“There is a large-scale rescue operation on the way and it is something that we haven’t seen in the area in my lifetime. It’s very traumatising for everybody.”
The Shandon Hotel in Dunfanaghy said some of its team members had been “directly affected” by the incident and was reserving spare rooms for first responders.
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