Cambodian riot police officers stand guard behind barbed wire as they block entry to Freedom Park, in Phnom Penh, Cambodia, Tuesday. Pic: AP.

WASHINGTON (AP) — The United States is calling for the release of Cambodian opposition officials held after a clash with security forces who prevented their followers from rallying in a public park.

The clash has heightened fears of political violence as Cambodia approaches the anniversary on July 28 of an election that the opposition says was rigged. Lawyers say at least six leading opposition members, including five lawmakers, have been charged with leading an insurrection movement.

(MORE: Cambodia locks up opposition lawmakers after protest violence)

State Department spokeswoman Jen Psaki condemned the violence that broke out Tuesday in a Phnom Penh park, causing injuries to security personnel and protesters.

Addressing a news briefing Friday, Psaki emphasized the importance of due process and called for the opposition officials to be released.

She also urged Cambodia to lift a ban on demonstrations.