Supporters take part in the "Justice March" in Istanbul on Friday, the 23rd day of the opposition’s protest over government detentions. (Chris Mcgrath/Getty Images)

THE 260-mile trek from Ankara to Istanbul is a grueling journey through rough terrain and scorching heat, but this hasn’t stopped more than 10,000 demonstrators from setting out on an ambitious march for “justice.” For the past three weeks, opposition supporters have walked for about 12 miles a day to protest President Recep Tayyip Erdogan’s unrelenting crackdown on dissent. Another leader would have looked at this groundswell of public outrage and refrained from persecuting critics. Mr. Erdogan went in a different direction: On Wednesday, Turkish authorities detained two foreign trainers and eight human rights activists — one of whom is the country director of Amnesty International.

Last month, we criticized Mr. Erdogan for the arbitrary imprisonment of Taner Kilic, the chairman of Amnesty International Turkey. By detaining his colleague Idil Eser, Turkey has done something unprecedented: Amnesty International tells us that this is the first time in the organization’s history that a director and a chair from a single country are behind bars at the same time. But Mr. Erdogan’s sweeping assault on dissidents and civil society is much larger than these two arrests: In April, it was estimated that more than 110,000 people have been detained since a failed coup attempt in 2016, and nearly 50,000 of these detainees have been charged with crimes. These numbers have continued to rise in the past three months.

Mr. Erdogan should not be allowed to imprison his critics with impunity. Though the march from Ankara is an encouraging sign that opposition forces are still able to organize, they cannot be expected to stand up to their repressive government alone. Foreign governments need to apply pressure on Turkey to end its attack on civil society and release those who were arbitrarily detained. This includes Mr. Kilic and Ms. Eser, and also Enis Berberoglu, an opposition parliament member, and many more journalists, human rights activists, lawyers and academics.

It was heartening to see the State Department immediately issue a statement rebuking Turkey for these unwarranted detentions, but the Turkish government has ignored such statements before. The United States needs to send a stronger message to Mr. Erdogan and other foreign leaders who continue to stomp on human rights. For a start, Secretary of State Rex Tillerson should put this issue at the top of his agenda when he visits Turkey on Sunday and Monday.