They now seem to be everywhere, moving in groups, their presence felt on every corner of Seoul.

"Running crews" -- groups of people that meet to run together at a specific time and place, otherwise known as run clubs -- have taken over Seoul's streets, parks and riverside tracks, reflecting the country’s growing obsession with one of the oldest and most basic human activities.

"There are tons of them now," said Patrick Hong, 26, a frequent runner on the trails around Gyeongbokgung. "When I'm running, I usually pass them real quick," he said.

The explosive rise of such groups here is an offshoot of the nationwide running boom, particularly among those in their 20s and 30s. What was once a solitary pursuit has now evolved into a social activity as groups of dozens to even hundreds enjoy the camaraderie and fun of running together.

Nowadays, recruitment for these groups is primarily done through social media. Prospective participants can test the waters with a "guest run" before fully committing to the crew. Some groups are pretty easygoing, welcoming runners of all ages and fitness levels. Others set higher bars, asking its members to keep up with a specific pace or show up for a minimum number of runs each month.

The buzz around running crews is evident in the numbers. The 7979 crew, Seoul's only public municipal run club led by the city government, has seen its membership triple from 300 last year to almost 1,000 this year, according to officials.

"These days, spots in city marathons fill up almost instantly — it used to take days before," a Seoul city official told The Korea Herald. "Our members are quite diverse, spanning from families with children to seniors. It's been catching on lately for sure."

But as more and more groups crowd the city's tracks, not everyone is buying into this trend. Some residents are growing increasingly frustrated with the crews’ ubiquitous presence, citing inconveniences and potential safety concerns.

Seoul's notoriously dense urban landscape often leaves little space for runners to maneuver. Even the Han River's parks, among the capital's most popular destinations for jogging and walking, have narrow lanes with no clear direction signs. Their proximity to bike lanes, with cyclists whizzing by, further contributes to the congested and hazardous conditions.

Lone runners have found themselves shoved aside or abruptly brought to a halt when a group of runners passes by. Meanwhile, some pedestrians have found the crews' demeanor to be irritating at best, intimidating at worst.

Eom, a woman in her 30s who lives in Yongsan-gu, has had several unpleasant encounters with running crews while walking her dog in Namsan Park. "Once, I was walking with my family and dog when a group of 5 or 6 runners passed us, loudly yelling 'Watch out!'" she recalled. "I was very upset. With all the available space on the street, why would they choose to run past us in such an in-your-face manner?"

Eom points out that she is hardly alone in her frustration. She notes that she has seen multiple posts in her local Karrot marketplace online community forum complaining about running groups, all of which received many likes and comments sharing similar experiences.

"I understand that most of them are not like that, but there must be a reason why so many people are upset about this issue," she said. "I hope these people straighten up and stop causing trouble on the streets."

Complaints about the alleged misbehavior of certain running crews have indeed been brewing on social media for some time. Some groups active on social media have come under fire for posting photos of members posing in the middle of running tracks or streets, seemingly obstructing pedestrians and traffic.

As tensions bubble up, some local governments have stepped in to take action.

On Tuesday, Banpo Sports Complex in Seocho-gu, Seoul, which offers five 400-meter lanes popular among runners, installed signs stating that groups should be limited to five people and maintain a 2-meter gap between runners on its running tracks. The facility is also making public announcements discouraging large group runs.

These measures, effective as of Tuesday, come after months of deliberation by the local government, according to officials.

"The recent surge in people running in groups has really crowded up the tracks," a management official at the Banpo Sports Complex told the Herald. "We’ve received a lot of complaints from residents, particularly families, about some runners going in the opposite direction or running shirtless, which they say makes them feel uneasy or unsafe. We had to do something about it,” he said.

A press release from the Seocho-gu office stated that these measures are intended to prevent certain groups, some of which charge members for running lessons, from taking up all the space and to create a safer environment for residents. It said, however, that these are not enforceable rules, but rather suggestions aimed at helping different groups get along.

When asked about the complaints, a Seoul city official in charge of the municipal 7979 run club told the Herald that they have no control over or responsibility for private clubs formed by individuals.

The official added that the city's run club has guidelines in place to minimize inconvenience to pedestrians, such as running in a single line and offering monthly safety lessons to raise awareness among its own members and other private crew members.

Lee Sang-wook, 33, who runs a tech startup in Seoul, has seen the city's running scene change up close.

Seven years ago, he started leading a running group based in Seongsu, east of Seoul and has been in charge ever since. Every Tuesday, Lee takes as many as 20-30 people on evening runs across the greater Seoul area.

"Running crews are all the rage now," Lee said. "More people want to join, and there are tons of active crews in Seoul. It's cool that people can now pick the crew that suits them best." He believes the best part of joining a crew is the bond it creates among people from all walks of life.

Lee is no stranger to complaints, having experienced occasional run-ins with annoyed pedestrians. But he wants people to see how runners are trying to do better.

"We always listen to what people say and use it to improve our rules," Lee said. He explained how the members of his crew stick to key principles like running in single file on narrow streets, breaking into smaller groups, keeping the noise down and following traffic rules.

In addition to their regular running activities, Lee and his group have also set their sights on making a positive impact in the community. They've turned their runs into clean-up missions, picking up trash and litter along the way -- a practice known as plogging. The group has also organized fundraisers to support visually impaired runners. In February, they worked with 12 other Seoul-based running clubs to deliver over 3,000 coal briquettes to households in need.

What concerns Lee most these days is the growing hate against running groups on social media and online forums, which he sees as more often based on sweeping generalizations than factual evidence. He notes how some local media outlets have added fuel to this sentiment by running inflammatory stories that portray running crews as public nuisances.

“I really hope there are more chances for different groups to come together and build a culture of mutual understanding,” he said. “We all agree that running is great for public health, and I think if members of the community can all sit down and talk, we’ll figure out a solution together.”