NetBlocks tell Euronews the extreme scale of the outage is "rare" and "worrying to see".
Unrest broke out in the Ethiopian capital, Addis Ababa, on Tuesday following the death of a popular singer.
Hachalu Hundessa, a well-known artist and activist, was shot dead on Monday. The motive for his killing is not yet known.
Hundesa had been a prominent voice in anti-government protests that led to a change in leadership in 2018.
Prime Minister Abiy Ahmed, who took office after the protests in 2018, led tributes to Hundessa on social media, describing him as a "magnificent and vibrant young artist".
Ahmed confirmed on Facebook that he is awaiting an investigation report into Hundessa's killing and appealed for calm in Ethiopia.
"We are at a time when we are keenly aware of the magnitude of the action and focus on activities in our country," said Ahmed.
Director-General of the World Health Organization, Tedros Adhanom Ghebreyesus, also expressed his "deepest condolences" at Hundessa's death on Twitter.
On Tuesday, internet service was cut off across the country, clouding reports of injuries and unrest.
"This is a whole country that has been taken offline," Alp Toker, CEO of NetBlocks, told Euronews.
"This is an effort to switch off communications both internally inside the country, but also to prevent communications with the outside"
NetBlocks also say the extreme nature of the internet shutdown in Ethiopia is rare. Their internet observatory found that the shutdown began at 06.00 UCT (05.00 CEST) on Tuesday, and the country remained offline as of Wednesday morning.
The disruption impacts affect cellular and fixed-line networks and are not connected to any known international connectivity issues or technical outage, say NetBlocks.
"It's quite worrying at the moment that so little information is going in or coming out of the country".
"It stops people from knowing what is going on in their country and becomes a source of misinformation of and by itself".
Senior United Nations officials have previously called on Ethiopia to put an end to internet shutdowns after online access was cut multiple times in 2019 without explanation.
Tensions in Ethiopia have continued after the government delayed this year's national election, citing the coronavirus pandemic.
There was no immediate sign of further protests in Addis Ababa on Wednesday and roads were empty.
Watch Seana Davis' Interview with NetBlocks in #TheCube in the player above.