The failure by Libya's government to keep order at the border crossings, vital supply routes for the Libyan capital, underlined the challenges facing the new leadership which took over from ousted leader Muammar Gaddafi.
"The crossing is now open," Abd Essalem Chandol, a Tunisian official, told Reuters by telephone from the main crossing point at Ras Jdir.
"Tunisian security forces decided to ... return to supervise the crossing again after adequate safeguards were put in place under the supervision of regular troops on the Libyan side," he said.
Tunisian authorities also re-opened the second, smaller crossing, at Dehiba further to the south.
"Dehiba crossing was opened suddenly this morning ... Some Tunisians have begun to cross into Libya," said a local resident who gave his name as Ali.