The arrest of Hossam Bahgat, one of Egypt’s leading investigative journalists, by Egyptian military intelligence on Sunday prompted an outpouring of concern, solidarity and outrage on social networks.

Mr. Bahgat was forced to “spend the night in military detention because prosecutors are yet to decide whether to issue a detention order or to release him,” according to Heba Morayef, the associate director of the Egyptian Initiative for Personal Rights, a prominent human rights group that Mr. Bahgat founded before turning his attention to journalism after the country’s 2011 revolution.