The Indian Army in Punjab has been put on alert amid spiraling terrorist violence in the troubled state that claimed at least 13 lives today, according to officials.

A curfew was imposed in the predominantly Hindu city of Ludhiana after five Sikh terrorists in police uniform, riding motorcycles of the same color used by the police, opened fire on a group performing calisthenics in a field, killing three of them.

The terrorists moved to other areas of the city, including a crowded vegetable market, firing indiscriminately at crowds, according to news agency reports, killing at least 10 persons and injuring many more.

Hindu protests erupted almost immediately in several towns, including Kapurthala, where one person was killed, and in Nakodar, where another person was killed by police gunfire, spurring the military alert.

In the state capital of Chandigarh, Chief Minister Surjit Singh Barnala called for communal unity in the face of the violence and dismissed Punjab's police commander. He gave no reason for his action in replacing K.S. Dhaliwal with Joseph Rebeiro, a former police chief of Bombay.

Ludhiana is the eighth town in Punjab to come under curfew during the past two weeks.

Sectarian violence in the Punjab escalated after the installation of a moderate Sikh state government in September and an earlier peace accord between mainstream Sikhs led by the Akali Dal party and Prime Minister Rajiv Gandhi.

Extremist Sikhs are seeking an independent nation, and sectarian conflict in Punjab during the past six months has taken nearly 200 lives.

Barnala, blaming today's attack on a "foreign-inspired conspiracy," called, with other Akali Dal leaders, for a statewide closure of shops in memory of the terrorists' victims.

A Hindu party, meanwhile, called for his resignation and for a protest strike against the killings on Saturday.