Spectators had crowded into a large sports stadium in Grozny when the planned Victory Day celebration turned into a scene of horror.

The powerful blast erupted under the reviewing stand where Chechen President Akhmad Kadyrov and other top officials were sitting.

The explosion tore a huge hole in the stand as spectators scattered in panic. Security agents say the bomb was hidden inside the concrete of the stadium, which is why it was not detected during a security sweep before the parade was to start.

The explosion also killed the leader of Chechnya's State Council and injured numerous other military and civilian officials.

The violence erupted as Russian President Vladimir Putin was attending Russia's main Victory Day parade, in Moscow's Red Square.

Mr. Putin blamed the Grozny explosion on on "terrorists" and said they could expect swift retribution. The somber-looking Russian leader met in the Kremlin with Mr. Kadyrov's son, who was in Moscow.

President Putin praised the assassinated Chechen leader, saying Mr. Kadyrov worked hard for the Chechen people.

The explosion appeared to mark a major setback for Mr. Putin, who has long maintained that the conflict in Chechnya is all but over.

The Russian president appointed Mr. Kadyrov to lead Chechnya several-years ago. The 52-year-old Chechen leader was officially elected to the post of president last October in a ballot that was criticized by international observers.