THESSALONIKI, Greece — Seven riot policemen and five PAOK fans were injured during clashes before and after Sunday's game with Olympiakos in the Greek league.

PAOK fans resumed clashes with riot police outside the stadium following the scoreless game, pelting them with rocks and bottles, erecting barricades and burning trash cans. Several arrests were made, police said.

Before the game, three riot police had been injured during an hour-long attack by PAOK fans who threw rocks, bottles and firebombs.

Fans, most of them without tickets, had gathered outside PAOK's stadium to ambush Olympiakos's team bus on arrival.

A heavy police presence prevented them from doing so, prompting fans to attack officers. A police vehicle was set on fire and three officers were slightly injured, according to police, who used copious amounts of tear gas to disperse the fans.

PAOK fans also attacked and smashed referee Anastassios Kakos's car.

Tear gas was also used inside the stadium when PAOK fans threw several projectiles at Olympiakos players who came onto the field to warm up. Some fans left their seats and came onto the field to get away from the tear gas in the stands.

Olympiakos players were eventually able to return to the field as the capacity 28,700 crowd finally settled down. About 1,000 riot police were inside the stadium, with hundreds more patrolling outside.

The game's start was slightly delayed.

Police concern about violent incidents, which often occur at Greek soccer matches, was heightened after the two teams played in a Greek Cup match last Wednesday.

The game was marred by controversial refereeing decisions that, PAOK claimed, robbed it of its chances to advance to the semifinals. Olympiakos won that game 2-0 in extra time, advancing 2-1 on aggregate.

The potential for violence has led to three prosecutors attending the game to report on any incidents.

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Associated Press Writer Demetris Nellas in Athens contributed to this report.