The Tigers had also fired on survivors of the bus bomb, police said.
The attack around 7.40 am local time that targeted the passenger bus killed 26 and injured over 60, the defence ministry said, as a ceasefire with the Tamil Tigers officially drew to a close.
The bus was travelling from the town of Buttala to Okkampitiya.
Tamil Tigers are still present in some parts of eastern Sri Lanka, though populated areas are now under government control.
The defence authorities appealed for blood type 'O' Positive and 'O' Negative which they said was immediately required at the Monaragala hospital to treat wounded civilians.
Blood donors were asked to go to the nearest blood bank if they could not travel to Monaragala.
In a second attack at around 9.45 am local time, three soldiers were wounded when the