Democratic Republic of Congo opposition presidential hopeful Moise Katumbi has been charged with hiring foreign mercenaries.
A government spokesman said he was plotting against President Joseph Kabila, Reuters reports.
Mr Kabila is nearing the end of his second term and he is constitutionally obliged to step down by December.
But there is growing political tension as it is not clear if he will relinquish power.
There have been several clashes between Mr Katumbi's supporters and the security forces in recent days.
Mr Katumbi described the charges as "grotesque lies", the AFP news agency reports.
But a guilty verdict would end his presidential campaign.
Government spokesman Lambert Mende said that he could now either be jailed or put under house arrest.
Mr Katumbi is a wealthy businessman who has a lot of support in the mineral-rich south-eastern Katanga province, and was at one time an ally of the president,
He has the backing of seven opposition parties for his run for the presidency, in an election which is scheduled for November.
The Constitutional Court last week ruled that Mr Kabila could remain in power after his mandate ends if elections are not held by the end of 2016.
Who is Moise Katumbi?
Moise Katumbi was governor of the south-eastern Katanga province for almost a decade.
In September last year he broke ties with the ruling party when he accused President Kabila, his former ally, of wanting to cling to power.
His popularity is partly down to his job as the president of a great source of Congolese pride - football club TP Mazembe.
They are Africa's reigning football champions, having won the African Champions League for the fifth time in November.