The Polish site Wirtualna Polska reported that an unknown person climbed the memorial for the victims of the Smolensk air disaster in the center of Polish capital Warsaw and is threatening to blow himself up.

The site also quoted a witness saying: "He has something that resembles an explosive device at his feet, and a fuse in his hands."

The police have cordoned off the area of the incident.

For his part, police spokesman Sylwester Marchak said: "The police are present at the scene, including the anti-terror team and negotiators. A security zone has been established covering the entire Piłsudski Square and the surrounding streets."

The Smolensk air disaster occurred on April 10, 2010, and claimed the lives of 96 people, most of whom were members of a Polish official delegation headed by President Lech Kaczyński, who were on their way to the village of Katyn in Russia's Smolensk region.

In conditions of poor visibility, the pilots decided to land, and the plane crashed just meters from the runway of Smolensk-North Airport.