Rodrigo Quintana, 25, leader of the opposition Liberal Party's youth branch was shot as police searched the premises in the capital Asuncion, party leader Efrain Alegre told reporters.
The interior ministry said in a statement that "the authorities are investigating the circumstances of the death, which is presumed to have occurred at the hands of a member of the National Police."
Police raided the party offices after activists took refuge there during a night of riots, Alegre said.
The unrest left about 30 people injured, including three lawmakers, according to firefighters and an opposition senator.
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