The reason for Yazdanfar's arrest is unclear, but the move has led to speculation that authorities are either planning to arrest Musavi or put him under house arrest.
The opposition "Green Voice of Freedom" website says the former prime minister has asked his office staff not to come to work until further notice.
Yazdanfar, the head of Musavi's bodyguard team, has reportedly worked for him for a number of years.
Here's a picture of Yazdanfar (in the white shirt) next to Musavi at a June protest in Tehran.
Yazdanfar's arrest comes just two days after Tehran Prosecutor Abbas Jafari Dolatabadi slammed Musavi for speaking out against the recent execution of five prisoners on terrorism charges and said the former prime minister is spreading lies. Musavi criticized last week's executions as unjust. Jafari Dolatabadi said Musavi's statement added yet another "crime" to an already long list and added that Iran's judiciary will take "appropriate action at the right time." He said many positions adopted by Musavi and reformist cleric Mehdi Karrubi are "criminal."
On May 16, 175 members of the conservative-dominated parliament called for the prosecution of Musavi and Karrubi before the one-year anniversary of the disputed June 12 presidential vote.
Iran's President Mahmud Ahmadinejad has said that opposition leaders should be punished over the unrest sparked by his declared victory in the presidential vote.
-- Golnaz Esfandiari