The activists are calling for the removal of nuclear weapons from Europe.
Three MEPs from the Greens political grouping have been arrested after breaking into a Belgian military airfield, according to the group's Twitter account.
The trio, including UK MEP Molly Scott Cato, Tilly Metz of Luxembourg and France's Michele Rivasi were among a group of activists who unveiled a banner on the runway of the base in protest against its hosting of US weapons.
The Greens tweeted a photo of the group calling for the withdrawal of US nuclear weapons from Europe.
The subject of US nuclear presence in Europe has become more sensitive in recent weeks after the US announced its withdrawal from a Cold War-era accord designed to limit the type of intermediate-range weapons that could threaten the continent. Washington claimed that Russia was in violation of the deal, an accusation that Moscow denied.