The law has become the latest flashpoint for those fearing growing Chinese encroachment on Hong Kong affairs, amid warnings it could contravene international conventions to which Hong Kong -- but not China -- is a signatory.

Opposition

Passage of the local version of the law is not expected until next year, and is sure to meet with concerted protest from the city's opposition pro-democracy movement.

Those protesters -- including well-known activist Joshua Wong -- were granted leave to appeal their sentences to Hong Kong's highest court earlier this month. That case will be heard in December.

Hong Kong politician Claudia Mo, an independent democrat who's a member of the Legislative Council, told CNN that if the law is enacted in the city and people are punished for booing the anthem, their "frustration and anger will grow."

"They will feel worse that they're getting clamped down, that the government is behaving in a rather authoritarian way."

She says the perception is the government "isn't trying to solve problems, just punish people who act against these problems."

This year marks 20 years since the handover of Hong Kong from British to Chinese rule in 1997, when, under the principle of "one country, two systems," Beijing promised to preserve the rule of law, freedom of speech and the right to protest for 50 years.

'Critical time'

Jeffery Au, a catering manager who was in the stands to see his team lose 1-0 to Lebanon, told CNN that now was a "critical time" for Hong Kong, as Beijing's hand in Hong Kong affairs seems more apparent.

Au, like others, fears a clampdown on booing is a sign of unwelcome political change.

"Of course (fans have the right to boo). It's a stadium, we can do what we want. If we like the song we will celebrate the song but right now (fans don't) really like the song."

He says fans will take whatever measures they can -- covering their faces with scarves or wearing masks or hats -- to obscure their identities and continue to boo the anthem.

The law change in China will "definitely" have an effect on fans in Hong Kong, Patrick Au Yeung, a sportswriter for a local football website says.

"A lot of reporters are filming especially the Hong Kong cheering section so (the fans) will try not to have their face on the TV. The booing has actually diminished since last Thursday in the (friendly) game against Bahrain."

He also says that the fans have the right to boo, but should separate sports and politics.

"I think everybody should respect their own national anthem, but I think Hong Kong fans should be concentrating more on the game than the political issues."

"The fans who boo have made their point now and I'm pretty sure that if there hadn't been such interest shown by the politicians and in particular the media, it would have stopped a long time ago," he said.

Sutcliffe also criticized what he termed an "orchestrated anti-booing rent-a-crowd" who, he alleged, were paid to attend the match for the purpose of confronting the booing fans.

"I don't know who these people are or who is paying them but they are clearly not there to watch the football," he said.

The Asian Football Confederation has threatened sanctions against the team if the booing continues.

Boos, jeers, offensive gestures

As the Chinese anthem was announced, the boos started and increased in intensity as the first notes of "March of the Volunteers" were played, a sizable number of people in the crowd, many wearing the red shirts of the Hong Kong national team, jeered and booed, gesticulating and waving their flags.

Pro-Hong Kong flags, including one which read "Die for Hong Kong" were waved. Stewards rushed up the stairwells in between the stadium seating sections, waving their hands at the booing fans but that wasn't enough to quell them.

Banners are supposed to be approved by the HKFA, an association spokesman told CNN.

Meanwhile, a counter group of fans, who support both China and Hong Kong, were positioned in a distant part of the stadium. They numbered around 80 and were located in an isolated part of the South Stand. Above where they were seated they had draped a Chinese flag.

Umbrella connections?

In amongst the Hong Kong supporters, there were a few signs of the Umbrella movement, the 2014 political movement which saw sections of Hong Kong blocked by protesters for months -- one older fan, a man in his 50s or 60s, wore a t-shirt with "I love HK" written on it, but with the heart replaced by an umbrella.

Many of those booing the Chinese national anthem have expressed support for Hong Kong independence, calls for which have been growing since the end of the Umbrella Movement pro-democracy protests in 2014.

Beijing has reacted angrily to any calls for Hong Kong to separate from China, with President Xi Jinping warning during a ceremony to mark 20 years of Chinese rule over the city that it was a "red line" for the central government.

Lawmaker Alvin Leung told CNN that there was no connection, nor coordination between the fans who are booing and the pro-democracy camp. However, he said that while his camp does "not welcome" the form of protest, watchers need to understand the rationale.

"In the past couple of years there have been growing issues, unhappiness against China, and the growing encroachment of Beijing on local issues.

"We need to understand why young people are not happy; (the booing) is just a reflection of their emotions."

Set-to

Not all pro-Chinese elements were sequestered in the stadium's South Stand.

During half-time an altercation sprung up in the ultras' section -- hundreds of red-shirted fans surrounded one or a couple of people for several minutes, yelling obscenities in Cantonese and gesturing at them with their middle fingers raised.

It was impossible to verify the identity of those being abused but fans told CNN they were journalists from Chinese state media CCTV. They were eventually hounded out of the section as fans sang, "we are Hong Kong" in English.

CNN was unable to reach CCTV for comment.

Hong Kong's South Korean coach, Kim Pan-gon, declined to comment on the effect the boos may have on his players' morale.

"I'm a football coach," he told CNN at the post match press conference. "Ask about football. I only know about football."