One man forced his way into the embassy but was roughed up and expelled by Pakistani security forces, the witnesses said.
Iran says that Pakistan, which it accuses of manipulating the Taliban, is responsible for the safety of the 11 diplomats and the journalist for Iran's official news agency, IRNA, missing since the Islamic movement seized the northern city of Mazar-i-Sharif last weekend.
A statement from the Taliban government, which controls virtually all major Afghan centres, denied that it was holding any diplomats but said it had arrested 35 Iranian truck drivers delivering ammunition to opposition forces.
Witnesses said the window of an embassy door was shattered as others in the crowd tried unsuccessfully to push their way inside. Iranian police then took up positions around the building and the crowd dispersed, they added.
Iran's special envoy for Afghan affairs told the country's international television service that the 12 were kidnapped when the city fell to the Taliban last weekend. He said they were safe and were being held in Kandahar.
Pakistan has said it would pass Iran's demands to the Taliban, but dismissed suggestions that it was responsible for the fate of the missing Iranians.
Relations between the Sunni Muslim Taliban and mainly Shi'ite Muslim Iran have been tense since the movement first emerged in force in 1994. Iran recognises the administration of the ousted president, Burhanuddin Rabbani, which also holds the Afghan seat at the United Nations. Pakistan, Saudi Arabia and the United Arab Emirates recognise the Taliban government.Reuse content