Police spokesman Zia Durani said militants used a new style of weapon in the attack in the southern province of Kandahar.
He said they threw a bottle packed with explosives which blew up when it hit the ground.
Mr Durani said the dead included the head of the provincial media centre and two policemen. He said seven other people were injured.
The incident is the latest in a string of attacks surrounding the Nowruz holiday.
The Taliban have threatened a campaign of violence to disrupt April 5 national elections.
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