Nabeel Rajab was detained yesterday for questioning.
Lawyer Jalila al-Sayed says Rajab was referred to the public prosecutor today and ordered to remain in detention another seven days pending investigation.
The Interior Ministry says Rajab was summoned because his tweet "denigrated government institutions."
Rajab wrote that security institutions are "the first ideological incubator" for Bahrainis who joined the Islamic State group fighting in Iraq and Syria.