Checks have been carried out at Brussels Central Station and in the European District.
Police spokeswoman Ilse Van de keere: "Everybody is being identified and frisked. People carrying suspect items are being arrested. "
So far, at least 115 people have been arrested.
Belgian rail says that at the request of the police some train services will no longer stop at Brussels Central. The protests are also impacting on public transport in the Belgian and Flemish capital. Metro station Park has been closed. No metros are stopping here.
Police are present in uniform and plain clothes.
Shortly after midday demonstrators clashed with police in the Montoyerstraat. 500 protesters were making their way to the Schumanplein, but police barred their route through the Wetstraat and protesters fled into sidestreets. A small group managed to get past police triggering the confrontation. Bottles, traffic signs and bomblets were thrown at the police.
Soon, the situation escalated further. Stones and paint were hurled at police that responded with watercannon. Press agency Belga reports the use of tear gas. Demonstrators were also kettled at the intersection of Regentlaan and Lambermontstraat after it became clear some had come to Brussels with violent intent. The US embassy is one flashpoint.
At one point demonstrators blocked the inner Brussels orbital. Police then went on to perform massive arrests of demonstrators. Some protesters were determined to destroy everything on their path.
As the demonstration ended protesters were made to identify themselves by police, hand over their Hi-Viz jacket and have their photograph taken.