BUCHAREST, Romania (AP) _ The politician who led Romanian coal miners on a violent march last month was sentenced today to 18 years in prison for previous violent demonstrations, national radio reported.

The Supreme Court lengthened Miron Cozma's sentence from 18 months in upholding his earlier conviction for illegally possessing firearms, undermining state authority and jeopardizing railway traffic, the report said.

The conviction stemmed from a rampage through Bucharest in 1991 that prompted Romania's first post-communist prime minister, Petre Roman, to resign.

About 5,000 coal miners from Romania's Jiu Valley commandeered trains and traveled to the capital on Sept. 24, 1991. Subsequent clashes left three people dead and nearly 300 injured.

Cozma was jailed in January 1997 for the violence, and convicted by a Bucharest court last June and sentenced to 18 months in prison _ most of which he served. He was released in July pending appeal.

In the latest labor violence, at least 200 people were injured in clashes with police when Cozma led 10,000 miners on a violent protest march across much of Romania last month, demanding higher wages and no layoffs.

The march stopped short of Bucharest only after Prime Minister Radu Vasile promised them wage increases of up to 35 percent if they come up with a plan to reduce losses at unprofitable mines by 20 percent this year.