Supporters of Turkey's main opposition Republican People's Party (CHP) leader Kemal Kilicdaroglu walk near Izmit, Turkey, on Monday with a giant Turkish flag on the 19th day of the "justice" march protesting a spate of arrests by the government. (Umit Bektas/Reuters)

The midday sun was roasting, their limbs were aching, and the protesters, from Turkey’s largest opposition party, still had 125 miles to march before Istanbul, their destination. The road had been hard: One elderly protester died of cardiac arrest, and another was hospitalized with heart spasms.

But spirits were soaring as they walked through Duzce last week, holding banners that said “justice,” to protest a spate of government arrests. A march that had seemed likely to fizzle or be stopped by the authorities had instead swelled in size since it left Ankara, the capital.

In recent days, pictures of the growing crowds have been passed around on social media, attracting newcomers. The spectacle has provoked an increasingly venomous response from officials, who have started associating the protesters with terrorist groups — a sure sign the demonstration has touched a nerve, organizers said.

The momentum has seemed like a breakthrough for Turkey’s browbeaten mainstream opposition, as its supporters resort to more creative and desperate tactics to meet what they say is President Recep Tayyip Erdogan’s stranglehold over the state’s institutions, such as parliament and the courts. But the 260-mile march, which is scheduled to end Sunday, has also risked a violent confrontation with the authorities as it approaches Istanbul, where thousands more people are likely to join the rally.

“We shouldn’t worry about it,” Kemal Kilicdaroglu, the leader of the opposition party, the Republican People’s Party (CHP), said about the potential for confrontation given the expected crowds in Istanbul. “Erdogan should worry about it.”

Republican People's Party leader Kemal Kilicdaroglu, center, and Socialist International Secretary General Luis Ayala, second from left, are flanked by marchers last week in Duzce, Turkey. The placard reads "justice." (Umit Bektas/Reuters)

The march began June 15, the day after the arrest of Enis Berberoglu, a CHP parliament member. He was sentenced to 25 years in prison for giving an opposition newspaper a video purportedly showing Turkish intelligence sending weapons to Syria.

Berberoglu, a former journalist, was the first CHP lawmaker to be jailed during a state crackdown on suspected terrorists and ordinary dissidents since a failed coup attempt last year, and his arrest was viewed by his party and others as an alarming escalation of the state’s campaign.

But it was just the last straw, said Aytug Atici, another CHP lawmaker. There had been outrage at the recent arrests of two teachers who had been on a months-long hunger strike to protest their dismissals from their jobs — action they took on behalf of hundreds of other academics who have been summarily fired since the coup attempt.

And there were the ongoing trials of some of Turkey’s most famous journalists, accused by prosecutors of treason, terrorism and other crimes, apparently for writing stories critical of the government.

“So many people are being thrown in jail. So many things anger me,” said Sabriye Demirici, 51, a doctor who participated in the march. She said that she was not affiliated with a political party and that the protest — which had grown to several thousand people last week, from the hundreds who had left Ankara — “shows that ordinary citizens are uncomfortable with what’s happening in the country right now.”

Some ordinary citizens, anyway. On the road, Erdogan’s supporters taunted the marchers, holding up the flag of his Justice and Development Party. The president dismissed the need for such a march, arguing that there were other ways for people to voice their complaints.

“Walking around with a placard that says ‘justice’ will not bring justice,” he said a few days after the march began. “The place to search for justice in Turkey is parliament,” he said, while also suggesting that it was wrong to march during the holy month of Ramadan and, more obliquely, that the protesters might be prosecuted.

The government has shown little patience for public demonstrations, which are tightly circumscribed because of Turkey’s ongoing state of emergency. At the same time, the CHP had a reputation for avoiding confrontations with the authorities, rather than taking fierce stands at the ramparts.

Kilicdaroglu, the party’s courtly leader, had faced derision for not holding street protests in April, after widespread reports of voting irregularities during a referendum that vastly expanded Erdogan’s powers. The complaints of inaction at a decisive moment had come from some in his own party.

“People criticized this a lot,” said Atici, the lawmaker, in ­praising Kilicdaroglu’s apparent change of heart.

Kilicdaroglu had also supported a government proposal last year to lift the immunity of members of parliament — paving the way for the arrest of leaders of an opposition pro-Kurdish party and then, last month, the imprisonment of Berberoglu. In an interview last week, he said he did not regret the decision, asserting that it was simply a part of his party’s platform.

This time, the party had been left with little choice, he said. “There is no parliament,” he said. “All the powers of the parliament have been transferred to Erdogan. There is no judiciary — it is completely inside the political authority.”

“Turkey is losing democracy and blood,” he said.

Along the march route last week, framed by rolling green hills, people appeared willing to put aside their misgivings about the CHP leader heading the march.

“I don’t think Kilicdaroglu is very effective,” said Merve Sahin, a 19-year-old student who attended the protest with her boyfriend, Gurkan Turan, a 21-year-old cook. They were not members of the party but attended “because something needs to be done,” Sahin said.

“We thought this was a start,” said Turan, rattling off a long list of grievances, including what he said was the mixing of religion and politics, the questions over the referendum, and the jailing of academics and others.

There had been surprising shows of support along the march route, from honking cars and clusters of well-wishers, even here in Duzce, a stronghold for Erdogan’s supporters.

“I feel great,” said Atici, who was full of energy on one particularly grueling stretch of road, as his fellow protesters were wilting.

He had been engaged over the past year in his own novel protest, refusing to cut his beard until the government lifted the state of emergency. He started growing it last July, and now, washed and combed out, it measured eight inches long.

The march, he said, had energized his party and stunned officials who were expecting that “the CHP will give it up after a few days,” he said. Buoyed by their success, some of his fellow lawmakers compared their leader’s stand, a little grandly, to Mohandas Gandhi’s famed march to the sea. It was not that, at least not yet.

But from the opposition’s perspective, it was a start. “We needed something like this,” Atici said.

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