KABUL—Rank-and-file Taliban fighters have been having too much fun in Kabul after seizing the Afghan capital without a fight last month, and the Taliban leadership has now issued a stern order to stop.

Thousands of young Taliban men from all over the country, many of whom have never been to a big city before, were deployed in Kabul after the Afghan republic collapsed Aug. 15. When not on duty, they sightsee, picnic and visit amusement parks, in a tangle of guns and turbans. Taliban fighters from elsewhere in Afghanistan have...

KABUL—Rank-and-file Taliban fighters have been having too much fun in Kabul after seizing the Afghan capital without a fight last month, and the Taliban leadership has now issued a stern order to stop.

Thousands of young Taliban men from all over the country, many of whom have never been to a big city before, were deployed in Kabul after the Afghan republic collapsed Aug. 15. When not on duty, they sightsee, picnic and visit amusement parks, in a tangle of guns and turbans. Taliban fighters from elsewhere in Afghanistan have also come to gawk at Kabul on tourist trips.

Top urban attractions for relaxing Taliban are Qargha lake, with its swan-shaped pedal boats, the Kabul zoo, and the Wazir Akbar Khan neighborhood, where visitors stroll on a grassy hill overlooking the city.

“I used to infiltrate in, to gather intelligence or to shoot people we were targeting,” said Shafiullah Masood, a 24-year-old from the province of Wardak, at the City Park amusement center with a gang of Taliban friends, waiting to go on the swinging pirate boat ride. “I’m happy that people are enjoying a peaceful life here now.”

A Taliban militant was amused by a deer in its enclosure at the Kabul Zoo.

Photo: bulent kilic/Agence France-Presse/Getty Images

The Taliban government’s Defense Minister Mawlawi Mohammad Yaqoob, a son of the Islamist group’s one-eyed founder Mullah Omar, wasn’t amused. In a recent speech, he castigated the fighters for going sightseeing, hanging out in big groups, making aimless trips to markets and to the Kabul airport, or exploring government buildings.

“Stick to the tasks you have been assigned,” he said in an audio message released Thursday. “You are damaging our status, which has been created with the blood of our martyrs.”

One key instruction was to stop taking so many selfies. Mawlawi Yaqoob was particularly annoyed that the Taliban foot soldiers pull out their phones to take pictures with leaders of the movement whenever they come across them. As these photos end up on social media, they compromise security by giving away locations and activities of the Taliban’s senior members, he warned.

Mawlawi Yaqoob also ordered the Taliban fighters to improve their attitude and appearance, bringing their beards, hair and clothes into line with Islamic rules. Taliban men sporting shoulder-length hair, stylish clothing and mirror sunglasses and wearing Servis Cheetah white high-top sneakers are a frequent sight across Kabul.

Taliban soldiers enjoying a boat ride in a dam on the outskirts of Kabul while taking selfies—a frowned upon activity.

Photo: Bernat Armangue/Associated Press

The Taliban fighters are also driving around too fast, Mawlawi Yaqoob complained, and not obeying traffic rules. The Taliban have appropriated fleets of Ford Rangers and Toyota Land Cruisers from senior officials of the former Afghan republic and from the army, government ministries and police.

“This is the behavior of the warlords and gangsters of the puppet regime,” he said, referring to the U.S.-backed government the Taliban overthrew. “If we continue to act like this, God forbid, we will lose our Islamic system.”

Since taking over Kabul, the Taliban have carried out a series of assassinations of political foes, including former members of the Afghan republic’s security services and people believed to be close to Islamic State.

In the western Afghan city of Herat, the Taliban on Saturday hanged four men from cranes positioned on four city squares that they said were part of a kidnappers’ gang, according to the owner of one of the restaurants near the scene of the hangings. Those were believed to be the first public executions since the Taliban takeover last month.

Since taking over Kabul in August, the Taliban have assassinated a number of political foes.

Photo: Paula Bronstein for The Wall Street Journal

Many in Kabul, a sophisticated city of six million people, deeply resent the Taliban for curtailing social freedoms that had been taken for granted for the past two decades, such as women’s access to education or work. The city’s lingua franca is Dari, an Afghan dialect of Persian, and many residents are unable to communicate at checkpoints with the Taliban fighters who often only speak Pashto, the main language of the group’s southern and eastern Afghanistan heartland.

“The country is finished. Everything is now in the hands of mullahs,” said a marketing executive for a cellphone company in Kabul.

So far, Mawlawi Yaqoob’s killjoy admonishments appear to have had only a limited impact on the Taliban foot soldiers, many of them still in their teens and bedazzled by novelties such as wheeled office chairs. Their encounters with the modern world have spawned an entire genre of mocking memes that have proliferated on Instagram.

Taliban fighters on Saturday enjoyed a round of wrestling, a popular sport in Afghanistan, in a Kabul park.

Photo: Paula Bronstein for The Wall Street Journal

On Friday, Ehsanullah, a 23-year-old fighter from the southern province of Helmand, was on his third visit to Kabul’s City Park, which offers a Ferris wheel, swing chairs, a haunted house, and a toy gun shooting range. He said that he had never seen such rides before.

“At first I was frightened of these rides, but once I had a go, it is no issue now,” he said.

Ehsanullah and fellow Taliban fighters at the amusement park have been told to hand over their Kalashnikovs and American-made M16 rifles at a special booth. There, a notice from the Taliban leadership instructs them to do so, saying that the presence of guns on rides scares women and children.

“Initially, the public was not familiar with us, our dress, our language,” added Ehsanullah, who goes by one name. “Gradually, people are starting to relax around us.”

Taliban forces patrolling in Kabul this month following the insurgent group’s takeover of the capital city.

Photo: stringer/Shutterstock

The Kabul Zoo, too, was filled with excited Taliban. A round of impromptu wrestling, a popular sport in Afghanistan, was taking place on a lawn which usually serves as a picnic spot. Fighters, along with some curious non-Taliban families, sat on the grass watching. Wrapping their arms around their opponent in a bear hug, the wrestlers would lift the loser off his feet and swing him around.

There was no contest over the most popular animal for the Taliban.

“I loved seeing the lion because we are also lions,” said Mohammad Amin, who hails from the province of Farah, sitting by a pond, where herons waded. “I was ready to be a suicide attacker. Now I’m happy that I am seeing Islamic law come to Kabul.”

Among the non-Taliban in the throng, not everyone was having such a good time. An employee of the presidential secretariat, at the amusement park with his young son, said he didn’t know if he still had a job. During his adult life, there have been bewildering, sudden, changes in regimes from the early 1990s, he said, ending with the Taliban back in power.

“My head is spinning,” he said.

A Taliban soldier posed with his gun on Wazir Akbar Khan hill overlooking Kabul.

Photo: Paula Bronstein for The Wall Street Journal

—Ehsanullah Amiri contributed to this article.

Write to Saeed Shah at saeed.shah@wsj.com