Algerian security forces, in a move likely to further increase tension in this North African country, have arrested the two leaders of the country's opposition Islamic Salvation Front: Abassi Madani and his deputy, Ali Belhadj. Mr. Madani last week vowed to wage a holy war against the regime of President Chadli Benjedid.Political sources said yesterday the arrests indicated a determination by the Algerian Army, a month after imposition of emergency rule, to wage a decisive battle against the fundamentalists. On Friday, Madani told followers at the weekly prayer service that the Front had broken off discussions with the government. Apparently in response to the remarks, demonstrators took to the streets Friday night and clashed with police. The most serious clashes occurred Saturday night, when automatic weapons fire could be heard in many Algiers neighborhoods. The government says 13 people were killed and another 68 wounded in clashes last week between demonstrators and police. The government imposed the curfew June 5 and postponed scheduled legislative elections, following street fighting between police and fundamentalists.