"La Marcha"--the march--is winding its way this week through the whitewashed villages that cling to the sierras north of Malaga on Spain's southern coast. Most of the land on either side of the narrow roads is part of large estates. At the villages where the hard core of 75 marchers stop to rest, meetings are held to denounce major landholders and demand the breakup of their properties.

"La Marcha," now in its second week, plans to cover 900 miles over 40 days and stage major meetings at 62 small towns and villages. It is organized by the increasingly powerful organization known as the farm workers' commissions, which has close links with the Communist Party and claims growing membership in the eight provinces that make up the Andalusia region of southern Spain.

In the village of Marinaleda in the adjoining province of Seville, a rival agrarian labor organization that calls itself the Farm Workers Syndicate held its convention last weekend. Two hundred delegates turned out to listen to a public reading of the convention's manifesto calling for the common ownership of land.

The syndicate, which is known by its Spanish initials, SOC, is mostly confined to Seville province, the most densely populated in Andalusia. The organization has built up a reputation for hardline agrarian agitation in the past years. The union has its roots in the once dominant anarchist tradition among Andalusian farmworkers.

Together the two unions appear determined to light a fire in the backyard of Spain's governing Socialist Party.

Agrarian unrest has a long history in the depressed southern region where large agricultural landholdings are the norm and grapes and olives are the staple crops. An estimated 160,000 workers in the south are classed as landless laborers dependent on seasonal labor.

Officials in Madrid concede the agrarian protests this summer and fall are more extensive and more radical than in previous years. They claim the Communist Party, which lost heavily to the Socialists in Andalusia in national elections last October, is exploiting the land reform issue to make up for lost ground.

The protests are timed to coincide with proposed agrarian reform legislation sponsored by the regional government of Andalusia, which is controlled by the Socialists. Leaders of both agrarian unions accuse the regional government of drafting a lukewarm reform bill that principally seeks to transfer the management of state properties from Madrid to the Seville-based Andalusia executive's agricultural department.

"The Socialists have already shown the guidelines of their reform to the landholders and have received the landholders' blessing," an SOC spokeswoman says. Commissions press secretary Jesus Bernabe claims 50 percent of productive land in Andalusia is owned by 2 percent of the landholders.

Both unions propose radical land reform. Bernabe cited an 18,750-acre estate between the cities of Cordoba and Seville that employs 14 farmworkers.

"If crops such as sunflower seed were sown, there would be work for 600 there," he says. The commissions and the SOC use as a rule of thumb the theory that 25 acres of productive land should employ one farmworker.

This week, more than 100 SOC members in eight villages staged a three-day fast to protest the detention of fellow union members in connection with the occupation of an estate near the village of El Coronil, close to Marinaleda. Eight SOC militants were arrested following the occupation and were released on bail after a week in jail.

Determined to reduce tension, the paramilitary Guardia Civil has used what the union spokesmen say is uncharacteristic leniency when ejecting protesters from a series of occupied properties in the past two months. The occupations have been peaceful and arrests have been kept to a minimum.

"Everyone is aware that Andalusia is a permanent powderkeg," said an aristocratic landholder who has extensive property south of Seville. He said he had withdrawn shotguns from his guards, "because the last thing I want is for anyone to lose his nerve and cause more trouble. Their orders are to call in the Guardia Civil."

The Madrid government has promised extra funds for Andalusia in the 1984 budget. In the meantime, local authorities have provided a police escort to keep the roads free for "La Marcha," and village mayors along the route have been providing meals and sleeping quarters.