South Sudanese government critic Peter Biar Ajak has been arrested, family sources confirmed.
Dr Ajak was reportedly arrested by the National Security Service (NSS) officers at the Juba International Airport on Friday.
His relatives said he was intercepted as he waited to travel to his home town, north of the capital Juba.
"Dr Biar was on his way to attend the Red Army Anniversary. However, we have information that he is being detained at the 'Blue House', the NSS holding centre," a relative, who wished not to be identified for safety reasons, said.
A recent ceasefire
"The motive for his arrest is not known but we, the family members, urge the arresting authorities to respect his rights and liberties," he said.
The relative described Dr Ajak as an ordinary citizen who posed no threat to anyone.
President Salva Kiir had earlier reportedly ordered NSS to crackdown on government critics to protect his administration's image.
Despite a recent ceasefire between the government and rebels, intolerance rules in South Sudan.