BRUSSELS — A man arrested alongside the Paris attacks suspect Salah Abdeslam has been formally charged in Belgium with participation in terrorist activities, in a case that sheds light on the scale and ambition of the Islamic State network that killed more than 150 people in the French and Belgian capitals in recent years.

The man, Sofien Ayari, a 24-year-old Tunisian, made his way to Brussels after crossing the Mediterranean in the summer of 2015, according to prosecutors, at the height of Europe’s refugee crisis.

Prosecutors say he became part of a Brussels-based network with at least nine safe houses, including two bomb-making shops, and which was planning many further attacks.

He was captured on March 18, 2016, hiding with Mr. Abdeslam, believed by investigators to be the only surviving on-the-ground perpetrator of the attacks that killed 130 people in and around Paris in Nov. 2015.