Belgian authorities asked people not to join a "March against Fear" Sunday after the deadly Brussels airport and metro attacks because of security concerns, suggesting the rally should be delayed by several weeks.

"We invite the citizens tomorrow to not have this demonstration," Interior Minister Jan Jambon said Saturday, citing security fears as Belgium remains on high alert after Tuesday's attacks which left 31 dead and 300 wounded.

Organizers said Sunday's march was meant to show that Brussels and the country at large refused to be intimidated by terrorism and that everyone stood together.

"This week, we, Belgian citizens have been attacked, in how we live, our customs, our rights, our liberty," a statement said.

"The first reaction in such events is to withdraw but on reflection, fear must give way to hope and the defence of our values." 

The march was due to begin at 2:00 pm at the central Place de La Bourse which, carpeted with flowers and tributes, has turned into a shrine to the victims.

Brussels mayor Yvan Mayeur said he recognised and shared the feelings of those who wanted to participate but given the absolute need for the police to concentrate on their investigations, it was best to delay the march.

"Let us allow the security services to do their work and that the march, which we too want to take part in, be delayed for several weeks," Mayeur told a press conference with Jambon at the national crisis centre.