A spokeswoman for the Emergencies Agency and witnesses said on Sunday they heard one blast followed by another five minutes later.
The headquarters sits on a busy thoroughfare in the centre of Kazakhstan's commercial capital.
"As far as we know, the blasts have hit the Otan headquarters," the spokeswoman said, declining to give details.
Alexander Pavlov, an Otan deputy chairman, confirmed that the headquarters had been hit by explosions but declined to comment further.
It was not immediately clear what had caused the blasts, though the area was quickly swarming with special forces and plainclothes officers, who cordoned off the three-storey building.
In September, the Otan party and its allies won all but one of the seats in parliamentary elections that the Organisation for Security and Cooperation in Europe said were marred by intimidation and fell short of democratic standards.
The lone opposition deputy has since resigned in protest against what he called a rigged and falsified poll.
In power since 1989, Nazarbayev wields almost unlimited power, and though he has reformed the oil-rich Central Asian state's ex-Soviet economy, Kazakhstan has never yet held a vote judged free and fair.