Around 50 demonstrators gathered in front of Phnom Penh municipal court Wednesday morning, where they stayed all day.
Police officials have said they want to question Mam Sonando in relation to the alleged movement in Kratie province.
The National Election Committee has so far received 18 complaints of irregularities during the campaign period.
City officials could not be reached for comment, but rights workers said they expect the 13 detainees will appear in court Wednesday.
Authorities say they want to arrest Bun Ratha for participating in an anti-government movement, alongside other suspects in Kratie.
Two demonstrations—one of factory workers and one of displaced city residents—were held on Monday.
Security forces have left a village in Kratie that was the site of the shooting death of a teenage girl by government forces last week.
Ten political parties kicked off campaigning across the country on Friday, in anticipation of commune elections June 3.
The delegation also met with UN officials in New York last week and were scheduled to talk with the US State Department Friday.
Tribunal spokeswoman Yuko Maeda said the judge had not expected the tribunal proceedings to last as long as they have.
Those arrested were Khat Sovann, 54, Pum Vannak, 53, Pon Soeurn, 34, and an unnamed fourth suspect.
The Cambodian People’s Party dominates the airwaves, and other parties have long complained that its hold in the media.