Hopes of making downtown Seoul dazzle more than Times Square have hit a setback with new guidelines to dim the digital billboards that light up the South Korean capital after a barrage of complaints.

The number of these sometimes huge electronic screens -- often curved and showing waving K-pop stars or leaping dolphins in 3D -- has soared, driving buzz on social media.

But not all locals are happy, particularly nighttime motorists bothered by the glare, with thousands of complaints in recent years prompting new recommendations from Wednesday to tone down the imposing installations.

The surge in digital billboards follows the designation of several areas as free advertising zones and a project to transform Seoul's central Gwanghwamun Square by 2033 into a "media gallery" to surpass Times Square in New York.

One billboard in Gwanghwamun Square is the size of four basketball courts and its 6K ultra-high resolution display is split across two sides of the building, creating a wrap-around effect.

The symbolic heart of the city has four large digital screens, bustling shopping area Myeong-dong has three and glitzy Gangnam -- synonymous with South Korea's economic and cultural strength -- boasts 17.

The colours "are so vivid that it feels like they are spreading out like waves," passerby Kim Hee-soo, 23, an art student, told AFP on Wednesday evening.

"I didn't realise it before, but the glass facades of the buildings also feel vibrant."

"Gwanghwamun Square will be reborn as a new media gallery that people around the world can enjoy," district mayor Chung Moon-heon said last year as he announced the project -- which boasts the slogan "Beyond your imagination, beyond Times Square".

Repetitive flashes

But the new guidelines recommend a maximum of 7,000 candelas -- a measure of luminous intensity -- during the day, half the level detected on some screens, reducing to 350-500 candelas at night.

The city also advises minimising high-brightness white backgrounds, using gradual instead of abrupt transitions in brightness, and avoiding repetitive flashing and intense strobe effects.

While the guidelines were drawn up before the Iran war -- which has prompted the government to seek to reduce energy use -- authorities hope they will also cut the installations' electricity consumption by 15 percent.

The recommendations are "a reasonable improvement that adjusts brightness beyond what is necessary to consider the readability of advertisements and reduce visual fatigue for citizens, while also enhancing energy efficiency," said Choi In-gyu, a local official.

Oh Se-min, 68, a retiree from a construction company, told AFP that he was not personally bothered by the billboards, but that safety comes first. 

"If drivers say they experience (glare), then it should definitely be addressed to prevent accidents," he said.

But Lee Youn-kyu, 55, who works in the shipping industry, wasn't convinced, saying that Seoul's billboards were less extreme than other global cities.

"We also can't ignore the overall convenience for pedestrians and the aesthetic value these lights add," Lee told AFP.

"I think it's important to find a good balance between all of these factors."