Oleh Tiahnybok, the leader of the nationalistic party All-Ukrainian Association Svoboda (Freedom), was questioned on Tuesday by investigators from the Ukrainian Interior Ministry's main investigations department as a witness to the events that took place near the Verkhovna Rada in Kyiv on August 31, 2015, the Ukrainian Interior Ministry reported.

Ukrainian Interior Ministry spokesperson Artem Shevchenko said Tiahnybok had been summoned for questioning to clarify some circumstances of the clashes between protesters and security forces on Constitution Square on August 31 and events preceding the unrest.

"To all questions concerning the organization of the rally, his participation in it and other issues significant for determining the circumstances of the incident, Oleh Tiahnybok refused to provide a testimony based on Article 63 of the Ukrainian constitution (an individual cannot be held accountable for refusing to provide a testimony or explanations with regard to themselves, their family members or close relatives identified as such by the law)," the Interior Ministry said.

Zorian Shkiryak, an advisor to the Ukrainian interior minister, said Tiahnybok would be summoned for questioning as a witness once again on September 14.

The investigation is currently assessing the status of each person involved in the August 31 unrest depending on the evidence it possesses.

It was reported earlier that Tiahnybok had filed an anti-defamation lawsuit against Ukrainian Interior Minister Arsen Avakov over the latter's accusations that Tiahnybok played some role in the clashes between protesters and security forces near the Verkhovna Rada on August 31.

Avakov said in response that he would file a counter-suit against Tiahnybok.

It was reported earlier that the Verkhovna Rada had passed a presidential bill on amending the constitution regarding the decentralization of power at the first reading by 265 votes on August 31.

Soon after that, protesters rallying against the bill started clashes with law enforcement officials near the Rada. As a result of the clashes and the explosion of a grenade thrown at law enforcement officials three National Guard members were killed and several others severely injured. A total of over 140 servicemen and civilians, including members of the press, were injured in the unrest.

Interior Minister Arsen Avakov blamed the Svoboda association and its leader Tiahnybok for the clashes. "I directly blame Oleh Tiahnybok and his Svoboda party. I consider it a crime rather than a political position," he said.

Tiahnybok said in response, "The fact that Avakov "appointed those guilty" so promptly indicates one thing, namely that he was the one who plotted the provocation."

The Interior Ministry had summoned about 30 people, including Svoboda leaders and active members, for questioning as part of a criminal investigation into the incident.