BREGA, Libya — Rebel forces routed troops loyal to Moammar Gadhafi today in a fierce, topsy-turvy battle over an oil port, scrambling over the dunes of a Mediterranean beach through shelling and an airstrike to corner their attackers.

The daylong fighting blunted the regime’s first counteroffensive against opposition-held eastern Libya.

At least 10 anti-Gadhafi fighters were killed and 18 wounded in the battle over Brega, Libya’s second largest petroleum facility, which the opposition has held since last week.

Citizen militias flowed in from a nearby city and from the opposition stronghold of Benghazi hours away to reinforce the defense, finally repelling the regime loyalists.

The attack began just after dawn, when several hundred pro-Gadhafi forces in trucks and SUVs mounted with machine guns descended on the port, driving out a small opposition contingent and seizing control of the oil facilities, port and airstrip. But by afternoon, they had lost it all and had retreated to a university campus five miles away.

There, opposition fighters besieged them, clambering from the beach up a hill to the campus as mortars and heavy machine gunfire blasted around them, according to an Associated Press reporter at the scene.