Nine people were killed and 28 others injured Friday during demonstrations against Quran burning by U.S. forces across Afghanistan, the country's Interior Ministry said.

A police car is seen on fire as people protest against the alleged Quran burning by U.S. troops in Herat, west Afghanistan, on Feb. 24, 2012. [Xinhua/Sardar]

"Some nine people including a policeman were killed Friday afternoon when protests to condemn Quran burning elsewhere in the country turned violent while 28 others including 12 police were injured," the ministry said in a statement.

According to the statement, protest demonstrations were held in 13 provinces including capital city of Kabul.

It said Friday's protest that erupted after Friday prayers have also damaged several cars, motorbikes, and other private properties, adding that the protests were continued until 5:00 p.m. local time in parts of the country.

U.S. president Barack Obama has apologized to Afghan president Hamid Karzai for the alleged Quran burning by U.S. forces in Afghanistan.

At least 25 people including two U.S. soldiers have been killed since violent protests began Tuesday.

Karzai has appealed for calm and urged the furious Afghans "not to resort to violence".