By Daren Butler and Tuvan Gumrukcu
ISTANBUL/ANKARA (Reuters) - Turkey's prime minister urged the head of the main opposition party on Friday to end a 425 kilometer (265 mile) march from Ankara to Istanbul in protest over the jailing of one of his lawmakers, saying justice "cannot be sought on the streets".
Kemal Kilicdaroglu, 68, head of the secularist Republican People's Party (CHP), set out on the march on Thursday after Enis Berberoglu was jailed for 25 years on spying charges. He was seen off by thousands of supporters and has garnered much attention in a country where government dominates the media.
Kilicdaroglu trudged along a highway outside Ankara on Friday dressed in dark slacks and blue shirt. Sunburned and wearing a cap, he carried a sign that said "Justice".
Rights groups and government critics, including members of Kilicdaroglu's CHP, say Turkey is sliding toward authoritarianism, citing a crackdown that followed last year's failed coup that has seen more than 50,000 people jailed and 150,000 sacked or suspended from their jobs.
"I advise Kilicdaroglu to desist from this act," Prime Minister Binali Yildirim told reporters. "Justice cannot be sought on the streets, Turkey is a state of law... Even if we don't like a court's ruling, we have to respect it."
Berberoglu was accused of giving the Cumhuriyet newspaper a video it used as the basis of a May 2015 report that alleged trucks owned by the state intelligence service (MIT)were stopped and found to contain arms and ammunition headed for Syria.
Berberoglu is the first CHP lawmaker to be jailed in the government crackdown, which has seen eleven members of parliament from the pro-Kurdish opposition party jailed.
Kilicdaroglu has called the arrest "lawless" and motivated by the presidential palace, a reference to President Tayyip Erdogan. His march, planned to end at the Istanbul prison where Berberoglu is being held, is expected to take more than 20 days.
"We have been, and will be, calling and defending justice, justice, justice, and democracy, democracy, democracy," he told reporters during his march. "No matter what they say."
Turkey's justice minister said Kilicdaroglu was trying to foment opposition to the judicial system.
"It is not possible to break the balance of the scales of justice by walking on roads," Bekir Bozdag said.
Erdogan acknowledged the trucks belonged to the MIT but said they carried aid to ethnic Turkmens battling Syrian President Bashar al-Assad and Islamic State, and not weapons for rebels.
Erdogan accused Cumhuriyet's editor-in-chief Can Dundar and Ankara bureau chief Erdem Gul of undermining Turkey's reputation and vowed Dundar would "pay a heavy price".
Last year, Dundar and Gul were sentenced to at least five years jail in a related case. The prosecutor is now seeking another 10 years for the two over the report on the trucks. Dundar is being tried in absentia after leaving Turkey. Gul remains in the country and free while his case is in process.
(Writing by David Dolan; editing by Ralph Boulton)