Kosovo police said Tuesday they have detained six opposition supporters on suspicion of involvement in a rocket-propelled grenade attack on parliament.

Police and prosecutors said in a joint statement that the five men and one woman have been accused of planning and committing the Aug. 4 attack that damaged the building's exterior but caused no casualties. Those detained are members or supporters of the opposition Self-Determination movement.

The statement said police seized several firearms and ammunition during Tuesday's detention operation, and all six people will remain in custody pending a judge's decision this week on whether to charge them.

The Aug. 4 blast was one of many that have rocked the fledgling nation amid tensions over the government's August 2015 agreement with neighboring Montenegro determining their common border. Opposition parties and even some members of the governing coalition are vehemently against the deal, arguing it cedes territory to Montenegro, but the government rejects this.

Self-Determination movement leader Visar Ymeri said the arrests represented "a classic act of persecution of political opponents."

The party has called for street protests Sept. 1, the day when lawmakers plan to vote on whether to ratify the border deal. Ymeri said the activists' detention represented a government effort to undermine the protest plans.

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An earlier version of this story was corrected to reflect that the attack involved a rocket-propelled grenade, not hand grenades; prosecutors did not say that the six detained people had admitted involvement in the attack; and the attack did not occur during a public protest.