By Marja Novak

LJUBLJANA (Reuters) - Police used water canon to break up rock-throwing protesters in Slovenia's capital on Friday after a rally against budget cuts and alleged corruption turned violent two days ahead of a presidential election.

Officers said it was the first time they had used that level of crowd control since the country's independence from Yugoslavia in 1991 and 15 people, mostly policemen, were injured.

Thousands of Slovenians took to the streets in Ljubljana and six other cities in the financially troubled euro zone country, the latest in a series of protests.

"I'm protesting against the government because they are destroying our state, our educational, health and social systems. We have to show that we are against them and that they are not untouchable," a 47-year-old engineer Ziva Brcar told Reuters.

Some members of the crowd started throwing granite blocks and firecrackers at police in the capital, where about 30 people were arrested, said police.

Slovenia, which joined the euro zone in 2007 when it was the fastest-growing economy in the bloc, was badly hit by the global crisis due to its dependency on exports.

It is now in recession and struggling with weak exports, a fall in domestic spending due to budget cuts and increasing unemployment, while local banks are hit by rising bad loans.

The conservative government of Prime Minister Janez Jansa this year cut all public sector wages and most social benefits in order to reduce the budget deficit to 4.2 percent of GDP from 6.4 percent last year.

It plans further budget cuts in 2013 to bring the deficit to about 3 percent of GDP, normally the maximum level allowed for euro zone members.

The country holds a presidential election on Sunday with former centre-left prime minister Borut Pahor, who advocates a need for cooperation between the government and the opposition, expected to beat the incumbent centre-left president Danilo Turk.

The president, who has a five year mandate, is mostly a ceremonial figure.

Prime Minister Janez Jansa on Thursday called upon citizens to keep demonstrations peaceful.

"The situation is serious but there is no need to give up. We have all the potential to overcome the crisis by ourselves," he said in a statement.

(Editing by Greg Mahlich)