"I will continue to fight until I get justice. Today, the court could have been a light for justice. The judge gave us darkness instead."
In early April, Hun Sen referred to an unnamed politician as a "strong leg," a term seen by some in Cambodia as offensive to women.
Mu Suh-Kwor has said the speech referred to her. She also denounced his remarks in another speech.
The court rejected her lawsuit in June, saying that it was groundless, but it moved ahead with the prime minister's countersuit.
Suh-Kwor, who was nominated for the Nobel Peace Prize in 2005 for her work against the trafficking of women in Cambodia, and Ho Vann, another politician, were stripped of their legal immunity last month and lost their seats in parliament.
Vann is accused of spreading false information and faces three years in prison.
|Hun Sen has said rights groups are interfering in Cambodia's affairs [GALLO/GETTY]
Suh-Kwor's case is the latest in a series of lawsuits and arrests targeting opposition voices.
Hang Chkra, a newspaper editor is serving a one-year sentence in Phnom Penh for writing about alleged government corruption.
In June Moeung Sonn, an opposition activist, fled the country after being given a two-year sentence for questioning a lighting system at the Angkor Wat temple complex, the country's biggest tourism drawcard.
Another opposition newspaper shut down after 10 years of publishing to avoid government legal action.
In June, the office of the UN High Commissioner for Human Rights in
Cambodia and the New York-based Human Rights Watch criticised the lawsuits against the politicians.
In a statement, the UN said the lawsuits undermined the constitutional freedom of opinion and expression.
Human Rights Watch said that Hun Sen had "a long history of trying to muzzle Cambodia's political opposition and undermine the independence of the legal profession".
Last month, the prime minister, a former Khmer Rouge fighter, criticised rights groups and foreign diplomats for interfering in Cambodia's internal affairs after they voiced concern about the removal of Suh-Kwor and Vann's parliamentary immunity.
Hun Sen has dominated Cambodian politics for more than two decades and won a landslide election in July last year.