OSLO, Jan 8 (Reuters) - Oslo police detained at least 27 people on Thursday after pro-Israeli and pro-Palestinian demonstrators clashed in one of the worst such outbursts in the Norwegian capital since the 1980s.

Shop windows in the city centre were shattered and police repeatedly used teargas to break up groups of activists demonstrating over Israel's crackdown in the Gaza Strip.

The violence started when about 1,000 pro-Palestinian supporters showed up at a rally sponsored by Norway's largest opposition party in support of Israel. Television pictures showed they burned Israeli flags and threw projectiles at police clad in body armour who separated the two groups.

"This has nothing to do with the situation in Gaza," Johan Fredriksen, chief of staff of the Oslo police, told the website of the daily Aftenposten.

"These people came to the protest with knives, bats and Molotov cocktails," he said, speaking about the pro-Palestinian side.

"You have to go back to the early 1980s to find a similar situation in Norway," Fredriksen said, adding that police were still concerned about security across the city.

Clashes also took place near the Israeli embassy.

Tabloid VG on its website said that Oslo was the site of a "street fight" with groups of men throwing rocks and bottles at police. One police car was trashed and one officer injured.

Before the flare up, some 15,000 people walked through the city centre for a peaceful, torch-lit protest, police said. (Reporting by Wojciech Moskwa and Richard Solem; editing by Michael Roddy)