Extinction Rebellion activists staged a protest next to the gardens of the Royal Palace in Brussels on Saturday, calling on the King to declare a climate emergency.
Organisers said the Belgian government had failed to act on climate change.
They demanded that the King, who traditionally avoids involvement in domestic politics and does not hold legislative power, acknowledge the gravity of the climate crisis and officially declares an environmental state of emergency.
According to police, 600 protestors amassed at the Place de Palais in Brussels. Later, police used water cannons to disperse them.
"The purpose here is to hold meetings as citizens where we speak about concrete solutions facing the ecological and climatic crisis," said Sarah, a spokesperson for Extinction Rebellion in Belgium.
"The idea is also to address the king because even if we are aware of the constitutional limits for him, we ask him to go outside these limits and disobey by symbolically declaring a state of emergency for the climate and the environment," she added.
Extinction Rebellion rose to prominence when activists brought central London to a standstill in 2018.
This year, protests have spread across the world in tandem with other environmentalist movements such as the Youth4Climate School Strike.
Action including "die-ins" and roadblocks have been seen from Melbourne, Australia to Berlin, Germany, as thousands.
The movement seeks to bring about systemic change through mass civil disobedience.
Extinction Rebellion's members, so-called "rebels", have the backing of hundreds of scientists and researchers who fear that catastrophic temperature changes will be irreversible within a number of years.
Belgium's carbon emissions per capita decreased by almost one third between 1980 and 2014, according to the World Bank.
The country's greenhouse gas emissions, however, remain significantly higher than the global average.