Turkey's martial law authorities enforced a 29-hour curfew on Istanbul that began at midnight to prevent the holding of a May Day political rally.

Labor leaders who had threatened to lead the rally despite the martial-law ban were arrested on Sunday. Official seek to prevent demonstrations that could trigger new violence between left and right that has taken 1,200 lives since Prime Minister Bulent Ecevit's social democratic government was voted into power 16 months ago.

Under the curfew, officials said, residents must remain indoors and security forces were ordered to shoot those without permits to travel.

Observers said the crackdown on the leftist unions was aimed at dissuading moderate members of Ecevit's government from withdrawing their support. Six ministers threatened two weeks ago to quit if Ecevi failed to act decisively against the violence.

The government imposed martial law in December in 13 of the 67 provinces, including Istanbul and Ankara, after Sunni and Alevi Moslems clashed in the southeast on Christmas Day, leaving 134 dead.

It extended martial law to another six eastern provinces last week, fearing that disturbances among Kurds in Iran would spread toturkey, where more than 6 million Kurds live.