Panic spread in Libya's western city of Misurata on Monday amid reports that up to 5,000 of leader Moamer Gaddafi's troops had amassed in a nearby town.
In a statement, a group of Misurata activists said the rebels had imposed a dawn-to-dusk curfew in response, DPA reported.
The activists said Gaddafi's fourth eldest son, al-Mutasim, was recruiting youth in the western town of Zlitan, which is part of the district of Misurata.
The opposition said many of the recruits were dressed in civilian clothes and driving civilian cars. Some were given gas masks, leading to speculation about their intentions and prompting rebel forces to boost check points against possible incursions.
Misurata, which is seen as a gateway to the capital Tripoli, has suffered from a shortage of medical and food supplies after over two months of shelling and fighting between rebels and Gaddafi's forces, with the opposition reporting that over 1,000 people have been killed there since fighting began mid-February.
Gaddafi's troops shelled parts of the city on Sunday, using Grad rockets and cluster bombs, the opposition says.
Meanwhile, Libyan deputy Foreign Minister Khaled Kaim expressed regret on behalf of the government over weekend attacks by regime supporters on foreign embassies in Tripoli.
The official said crowds angry at Saturday's NATO attacks on Tripoli had overwhelmed the police.
The NATO attack reportedly killed one of Gaddafi's son and three of his grandchildren.
There were reports that the British embassy, evacuated due to the conflict in Libya, had been destroyed and the US embassy set on fire.