About 1,300 were believed to have been sheltering in the building, with just 150 survivors having staggered out of the rubble immediately after the attack.
Sources said the bulk of those in the theatre had been hiding from the shelling under the stage and that rubble was blocking their exit.
There are no emergency services operating in Mariupol and the intense fighting and shelling near the theatre has prevented rescue attempts. The theatre was bombed by plane despite the word 'children' having been painted on the ground in white, The Guardian reported.
In a statement on Friday morning, a spokesman for the city council said they were able to share a new death toll from the tragedy.
The statement said: "Unfortunately, we start this day with bad news. From eyewitnesses, information appears that about 300 people died in the Drama Theatre of Mariupol as a result of the bombardment by a Russian aircraft. Until the last, I do not want to believe in this horror. Until the last, I want to believe that everyone managed to escape. But the words of those who were inside the building at the time of this terrorist act say otherwise.
"The Drama Theatre in the heart of Mariupol has always been the hallmark of the city. A place of meetings, dates, a point of reference. 'Where are you? I'm on Drama'. How many times have we heard or said this phrase: 'On the Drama'.
"Now there is no more Drama. In its place, a new point of pain for Mariupol residents appeared, ruins that became the last refuge for hundreds of innocent people." (IANS)
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