An Interior Ministry spokesman said a gas bottle exploded in a traffic police booth set ablaze by protestors during the anti-US demonstrations in the city.
The violent protests began in Kandahar and two adjoining districts of Panjwayi and Dand on a third day of violent demonstrations.
The protests, which started on Friday in the northern city of Mazar-i-Sharif, have claimed the lives of at least 22 people including seven foreign UN staff. On Saturday, 10 people were killed and more than 80 injured in clashes with security forces in Kandahar.
Afghan President Hamid Karzai has repeated his call to the White House, Senate and United Nations to bring to justice Terry Jones, the pastor who masterminded the burning of the Koran, Islam's Holy Book.