Belgian police and soldiers on Thursday sealed off part of central Brussels after finding gas canisters in a car driven by a man known to have been radicalised, officials said.
Belgium remains on high alert as it prepares to mark the first anniversary of last year's Islamic State-inspired attacks on the Brussels metro and airport which killed 32 people.
Prosecutors said the man was detained and is being questioned. Reports said he had previously gone to Syria but officials did not confirm that.
"The car was stopped because the driver jumped several red lights. The police then noticed several gas canisters in the boot and rather than take any risks, they called for help from the army bomb squad," a police spokeswoman said.
Local mayor Charles Picque told AFP the driver was "someone potentially dangerous" who was listed as radicalised.
"When you put it all together... and that there were gas cannisters in the boot, which he did not want to open, then obviously you have to be prudent," Picque said.
The Brussels prosecutors' office said the bomb disposal squad carried out several controlled explosions but found "no detonating mechanism or other explosives in the car."
The security perimeter, in force from mid-afternoon, had also been lifted, it said, without giving any further details.
Local RTBF television said the driver was aged 27 and had gone to Syria in 2014 and been arrested on his return to Belgium.
He was subsequently sentenced to five years in prison, suspended, at a trial of recruiters of foreign fighters for the Islamic State group in Syria.
The trial was of top jihadist recruiter Khalid Zerkani, who was sentenced in 2016 to 15 years in prison for recruiting dozens of people, including several who became key suspects in the Brussels and Paris attacks, to wage jihad in Syria.
Belgium is one of the largest sources of foreign fighters going to Syria and Iraq.
Thursday's incident sparked immediate concerns after French police apparently foiled an imminent attack in September when they detained several women driving a car filled with gas canisters next to Notre Dame Cathedral in Paris.
The security perimeter was set up in the Porte de Hal neighbourhood, near the main Midi rail station in an area that is famous for a medieval fortress which attracts tourists.
Nearby buildings were evacuated and public transport either halted or diverted, causing rush-hour chaos.
The Brussels prosecutors' office, which confirmed the driver was known to police, said he was now being held for questioning to determine what his intentions were.
"It is absolutely too early to say that the driver had criminal intentions. All speculation in this sense is premature," a statement said, adding that the identity of the driver could not be released while the investigation is underway.
Attacks carried out by home-grown, IS-inspired jihadists on March 22 last year rocked Belgium and sparked a major security clampdown.
The same jihadist cell also played a key role in the November 2015 Paris attacks which claimed 130 lives.