Belgian police stopped a coach coming from France on Thursday after a passenger reported a "terrorist" conversation about a possible attack, officials said, according to the AFP news agency.
The bus, run by the company Flixbus and operating a service between the northern French city of Lille to Brussels, was halted in the town of Wetteren, next to Ghent, a spokeswoman for the East Flanders prosecutors' office said.
Police arrested three passengers and were to question all those on board to determine if the suspect conversation was substantiated, she said.
A spokesman for Justice Minister Paul Van Tigchelt said the passenger had overhead a suspected "terrorist conversation" and alerted police who stopped the coach and inspected it.
The federal prosecutors' office said it was not treating the incident as a terrorist case for the moment.
The terrorist threat level in Belgium was raised in October from two to three, the second-highest level and considered "serious". Belgium's official Coordination Unit for Threat Analysis said on Thursday it was not raising the national terrorism alert level above its current status.
The move in October followed an attack in the Belgian capital in which two Swedish nationals were shot dead, and another injured, by a radicalized Tunisian.
The attacker was shot dead by Belgian police on October 17, the day after the attack.
A week after that attack, a Palestinian Arab asylum seeker was arrested in Brussels after he visited a migrant aid organization where he allegedly talked about planning a suicide attack.
Brussels was hit with a major terrorist attack in 2016, in which 32 people were murdered by Islamists.
Reports following the 2016 bombing indicated that Belgium received advanced warnings of the terrorist attacks but did not act on those warnings.