Turkish antigovernment protesters battled on Tuesday with police in Istanbul near Taksim Square, the epicenter of nationwide demonstrations. Mathias Depardon for The Wall Street Journal

ISTANBUL—On an elegant street here flanked by embassies and Ottoman-era townhouses, torn-up paving stones and contorted shards of metal rise six feet high, the detritus of demonstrators' running battles with Turkish security forces.

Scenes from Tuesday night's demonstrations in Istanbul. Thousands have been taking to the streets amid the biggest anti-government protests in Turkey in decades. WSJ's Joe Parkinson reports

Each night since antigovernment protests erupted across Turkey five days ago, thousands of protesters have come here to man these makeshift fortifications, which also include overturned garbage cans, burned-out vehicles and broken glass.

They say they will guard the area for as long as it takes to stop security forces from moving up the street to enter the adjacent Taksim Square, the epicenter of nationwide protests against what critics say is creeping authoritarianism of Prime Minister Recep Tayyip Erdogan.

Barricades in Istanbul

Mathias Depardon for The Wall Street Journal

Turkish protesters say they will man barricades for as long as it takes to stop security forces from entering Taksim Square, the epicenter of nationwide protests. Photo: Mathias Depardon for The Wall Street Journal.

This tree-lined area called Gumussuyu, or "Silver Water," which winds sharply downhill toward Istanbul's Bosporus Strait, has emerged in recent days as one of several battle zones in a city vying to host the 2020 Summer Olympics.

The demonstrations—by a broad cross-section of people, dominated by the young and educated—have been mostly peaceful. But clashes have raged between more radical protesters and police in more than a dozen cities, leaving two people dead and thousands injured, says the Turkish Medical Association.

Some of the protesters who guard the area represent a smaller and more radical dimension of the movement largely unprepared to accept any resolution where the popularly elected Mr. Erdogan stays in power.

As Mr. Erdogan traveled abroad on Tuesday, his deputy struck a conciliatory tone, apologizing for the government's initial violence, and Turkish markets reversed some of the losses of recent days in a sign tensions are abating. Mr. Erdogan has won three successive elections and faces no effective political opposition. Still, the presence of a minority of more militant protesters could hurt efforts to broker a peaceful resolution.

"We built these barricades to defend the square. This is the only thoroughfare where the police could bring lots of heavy vehicles or water cannon to clear the protesters," said Can Ozdemir, a hotel worker who said he had come to the zone every night since protests began. "As long as we hold it, they can't shut the protest down. We can keep the pressure on them."

Just a few hundred yards up the hill in Taksim Square itself, a more festive atmosphere reigns.

Mask-clad girls dance to Turkish folks songs blaring from tinny speakers. Abandoned buses, caked in antigovernment graffiti, have become makeshift climbing frames for young children. Protesters wielding trash bags help municipal services to keep the area tidy. Tourists snap vacation photos. The cafes on the square do a bristling trade.

"We're grateful for the people who are helping us defend the square. The longer we stay here, the more chance we have to change things," said an architecture student gathering the names of volunteers to help maintain the encampment.

Not everyone is so sanguine. At the Opera hotel on Gumussuyu's main thoroughfare, visitor numbers have plummeted in what should be peak season. In the hotel's top floor restaurant, staff complained that they have been trapped inside until sunrise because of the volume of the tear gas settling in the area.

"But we keep the door unlocked all night to shelter protesters who are choking," said a waiter who gave his name as Umut.

On Tuesday night, the now familiar routine around the Gumussuyu barricades replayed.

As darkness fell, protesters streamed down the hill, many shouting antigovernment slogans. Shortly before midnight, demonstrators faced off against hundreds of security forces stationed on the road to the 19th-century Dolmabahce Palace.

As protesters chanted, the police fired volleys of tear gas to beat them back. Some protesters threw rocks or pieces of paving stones, before retreating. Others offered each other a milky anti-acid solution to ease the gas's burn. Volunteer medics wearing white coats and workmen's hat marked with red crosses helped protesters. Some vomited on side streets, others needed emergency attention.

"I volunteered to help here because I believe in the cause. The government has to go," said a man wearing a stethoscope and surgical gloves. "I'll spend as time in the Square in the day and come here at night. I'll be here as long as it takes."

Write to Joe Parkinson at joe.parkinson@wsj.com