KAMPALA, Uganda (AP) — A child was killed and eight other children were seriously injured on Wednesday in an explosion in a suburb of the Ugandan capital, police said.

The explosion happened Wednesday at a school playground in Kampala where children were playing with an explosive device that they did not recognize, police spokesman Fred Enanga told The Associated Press. The device was a suspected hand grenade, he said.

The eight injured children were in critical condition in a hospital, Enanga said.

He said he believed the incident had nothing to do with the presidential election scheduled for Thursday.

Tensions have been rising ahead of the voting, which pits long-time President Yoweri Museveni against seven challengers.

Museveni's main rival, Kizza Besigye, has said he doesn't believe the elections will be free or fair.

Ahead of the polls, there has been a heavy security presence in Kampala, with heavily armed police patrolling the streets and armored vehicles parked at key junctions.

Museveni, who took power by force in 1986, has vowed to "smash" those who threaten national security during and after the elections.