Manuel Colom Argueta, a prominent leftist politician and former mayor of Guatemala City, was killed by unidentified gunmen yesterday morning in the Guatemalan capital.

Argueta, 47, who was machinegunned while driving to work, was the second major opposition leader killed in Guatemala this year in growing political violence. Alberto Fuentes Mohr, an opposition member of congress and former foreign minister, was assassinated in a similar manner on Jan. 25.

Colom Argueta, the popular mayor of Guatemala City from 1970 to 1974, was considered by many to be the best hope of the combined opposition to lead a future government, ending a succession of military-dominated regimes.

In an interview published recently in a Guatemalan newspaper, Colom Argueta, commenting on the Fuentes Mohr killing, said the "extreme right wing has changed its technique" for maintaining power in Guatemala, "and they now use political assassination in broad daylight."

The political party Colom Argueta headed, the United Front of the Revolution was officially registered by the government's electoral commission last Friday, eight years after it first applied.

The refusal of successive military-controlled governments in Guatemala to register new opposition parties, thus denying them the right to present candidates during elections, has been a traditional method of maintaining power. In a campaign promise last spring, President Gen. Romeo Lucas said that all opposition parties that could meet member qualifications would be registered.