Princess Marie-Esméralda of Belgium was reportedly arrested in London during the Extinction Rebellion protests.
The 63-year-old daughter of King Leopold III and the Princess of Rethy is said to have been taken away by Trafalgar Square officers then released without charge on Friday morning.
A 'released under investigation' document circulating online contains the princess' s name and is listed in a £ 6m property in Camden, North West London.
More than 1,100 Extinction Rebellion protesters, who have gone to London for two weeks, have been arrested since Monday.
They have been camping out across the capital, including Trafalgar Square, blocking major roads.
Back in April the group brought some of London's busiest routes to a standstill for more than 11 days.