A Congolese opposition candidate has been summoned to court in the city of Lubumbashi as Human Rights Watch denounces the arrests of his supporters.

Moise Katumbi, who last week announced he would be an opposition candidate for Congo's November election, appeared at a closed hearing Monday in Lubumbashi, after a summons Saturday on accusations of hiring mercenaries.

The justice minister last week ordered investigations into allegations that Katumbi hired mercenaries. Several of his security guards were arrested in March.

Human Rights Watch Monday condemned arrests since April of at least 27 of Katumbi's supporters.

Katumbi, Katanga ex-governor and president of one of Africa's top soccer teams, TP Mazembe, resigned from the ruling party in September 2015, alleging efforts to delay elections.