The People's Democratic Party (HDP) and the Democratic Regions Party (BDP) issued a joint statement condemning the assassination.
According to an AFP photographer, entry to the street where the office is located was blocked by riot police, who deployed armoured cars and anti-riots vehicles around the building.
Among those arrested was HDP Beyoglu branch Co-chair Rukiye Demir, Today Zaman reported.
DIYARBAKIR, Turkey Turkish security forces have killed 18 Kurdish militants in southeast Turkey, the military said on Saturday, in the latest clashes to blight a region still reeling from the collapse of a two-year ceasefire last summer.
A police spokesman said officers spoke to the building security team and building owners before implementing a "proportionate policing response".
His HDP party passed the 10% in the November snap elections and won 59 seats in the Turkish parliament.
Erdogan and the government accuse the HDP, parliament's third-biggest party, of being an extension of the PKK, which has fought a three-decade insurgency for greater Kurdish autonomy in the southeast and which is considered a terrorist group by Turkey, the United States, and the European Union.
Ankara has been stepping up its military operations on the border with Syria and Iraq since December.
In a typically tough address Wednesday, President Recep Tayyip Erdogan vowed that Turkey would press on with the "anti-terror" operations for the sake of all its citizens.
Dismissing any sympathy for the Kurdish armed resistance as "terrorism" is a clear indication of the government's failure to realize its role in forcing the Kurdish people to empower themselves in other realms than just the political one.
"This is slaughter. This operation is not against the rebels".
Since mid-August, the Turkish state has imposed curfews on several Kurdish towns and districts leaving people with no access to medical aid, with no food and drinking water and forced to keep their killed family members in makeshift freezers because they can not take them to the hospital or bury them.
