Indonesian police arrested five people, including a leader of a hard-line Islamist group, for suspected treason on Friday before a protest by thousands of Muslims demanding the sacking of Jakarta’s Christian governor, a police spokesman said.

Religious and political tensions have been running high ahead of the second and final round of the Jakarta election on April 19. Incumbent Basuki “Ahok” Tjahaja Purnama, who is on trial for blasphemy, is competing against a Muslim candidate.

Jakarta police spokesman Argo Yuwono told Indonesia media that five people had been arrested early on Friday, including Muhammad Al Khaththath, a leader of the Islamic People’s Forum (FUI), a hard-line group that organized Friday’s rally.

“We are charging them with conspiring to commit treason,” Yuwono told broadcaster TVOne.

“There were several findings, including a plan to occupy the Indonesian parliament,” he said.

Thousands of protesters gathered at Jakarta’s grand mosque on Friday as police and military personnel blocked off roads leading to the presidential palace in the city center.

Muslim protests have targeted Purnama, an ethnic Chinese Christian, on trial for allegedly insulting the Quran in connection with remarks he made about how people should vote. He denies the charge.

Purnama is backed by President Joko Widodo’s ruling party and the sudden rise of fringe, hard-line Muslim groups during the election campaign has raised questions about whether they are being used as political pawns and destabilising Indonesia, a secular nation with the world’s largest Muslim population.

Friday’s rally is the latest in a series that have tested religious and ethnic tolerance in Indonesia. An estimated 20,000 people from various groups were expected to attend, police said this week.

Purnama remains popular for his efforts to cut red tape and ease Jakarta’s chronic traffic congestion and flooding. However, he faces a tight race with rival Anies Baswedan, a former education minister.