Beijing’s liaison office in Hong Kong has warned that the city will have no future if radical anti-government protesters return to chaos and violence at a time requiring unity to fight the coronavirus crisis.

Having just recently asserted its authority to comment on the city’s affairs, dismissing cries of “interference” by opposition politicians who questioned its powers, the office issued a statement on Saturday lambasting those who staged illegal protests on Labour Day and hurled petrol bombs at night.

Just hours before three suspected home-made bombs were found in an abandoned school, Beijing’s representative office accused opposition politicians of encouraging young people to commit crimes in the name of fighting for democracy.

“We strongly condemn extreme radicals for staging unlawful gatherings again during the Labour Day holiday, disturbing shops and throwing petrol bombs,” a spokesman said. “People around the world have set aside differences in fighting the pandemic. It was only in Hong Kong that the extreme radicals have continued to incite people to engage in violent activities and threatened to plant bombs in public places. If we appease them, can Hong Kong have a future?”

While major protest chaos expected on May 1 did not materialise, with riot police out in force, a 15-year-old was arrested for hurling petrol bombs in Mong Kok. Officers enforcing a ban on public gatherings also dispersed protesters with pepper spray at New Town Plaza shopping centre in Sha Tin.

The violence and the “if we burn, you burn with us” mentality had undermined the rule of law and freedom, the liaison office said, marking the fifth time since April that it has spoken out in this manner.

“We noticed that the suspect arrested last night was only 15 years old. We want to ask the politicians, when you told young people that their life can be enriched by their criminal records, where are your children? How can you sacrifice the bright future of a generation like that?” the spokesman said.

According to police, the 15-year-old was held after he and three men threw the petrol bombs and some plastic tubes with protruding nails at the junction of Boundary Street and Nathan Road in Prince Edward. Officers found on him another petrol bomb, a lighter, seven glass bottles and two plastic tubes.

The teen was arrested for arson and “possessing anything with intent to destroy or damage property”, while the three other men, thought to be aged between 16 and 30, escaped.

More bomb materials were found in a related search of a private flat on Tai Po Road, and the 46-year-old owner was arrested for possession of an offensive weapon. Among the items seized were 3.6 litres of fuel, an expandable baton, glass bottles, lighters, pumps, helmets, torches, gas masks and yellow vests.

Police said they handed out 18 fixed-penalty tickets for breach of social-distancing rules throughout Friday, 11 of which were issued at New Town Plaza.

On social media throughout the week, protest organisers had called for flash mobs to assemble in five areas on Friday, but eventually said they only meant to encourage patronage of the so-called “yellow” shops and restaurants. But the strategy demonstrated the opposition camp was intent on hijacking the economy with politics, which inflicted damage on any shop that did not support the protesters, the office spokesman said.

The office again slammed opposition lawmakers for blocking the work of the Legislative Council with filibustering at a time when the city was suffering financially.

Last month, Beijing’s Hong Kong and Macau Affairs Office and the liaison office accused Civic Party lawmaker Dennis Kwok of “misconduct” by paralysing Legco with delaying tactics.

In Saturday’s statement the liaison office said the opposition bloc had turned into a “collateral damage camp”.

The term refers to a recent article by Benny Tai Yiu-ting, a professor at the University of Hong Kong and a leader of the Occupy Central movement in 2014.

In the piece titled “Ten steps of real collateral damage”, Tai predicted what would happen in 2021 and 2022 if opposition lawmakers became the legislative majority after the September elections – they would vote down the government’s funding requests, ultimately triggering the dissolution of the legislature and the resignation of the chief executive.

That would be followed by Beijing declaring a state of emergency in Hong Kong, heightened protests and eventually sanctions by foreign governments on the central government.

“The opposition camp engaged in political collateral damage in Legco … and even advocated the so-called ‘Ten steps of real collateral damage’,” the spokesman said. “They are pushing Hong Kong to a bottomless abyss.”

The office noted Hong Kong’s economy and global reputation had suffered since last year. In March, Hong Kong was knocked off the top of the Washington-based Heritage Foundation’s index ranking of the world’s freest economies, a position the city held for 25 straight years. The economy contracted last year for the first time in a decade, while unemployment had now reached a 10-year high, the spokesman said.

Opposition lawmakers remained defiant, saying the liaison office had failed to listen to Hongkongers demanding respect for the city’s autonomy, while pro-establishment legislators suggested Beijing was clearly growing increasingly worried about the city, and that local officials should be speaking up as well.

“It did not reflect on the fact that Beijing was partly to blame for triggering the anti-government movement, as it supported the now-withdrawn extradition bill,” Civic Party leader Alvin Yeung Ngok-kiu said. “On yellow shops, I just want to say that the freedom to choose is the backbone of capitalism, and we are just encouraging people to patronise shops owned by shopkeepers with similar political views.”

But pro-Beijing lawmaker Leung Che-cheung said the liaison office’s statement showed that the central government would be tough on illegal acts.

“The city’s government needs to make a clear stance on this too,” Leung said. “It also shows that Beijing is very concerned and worried about Hong Kong. The protests last year were destructive. If we repeat what happened, it will be difficult for the city to recover.”

Pro-establishment lawmaker Horace Cheung Kwok-kwan said: “Beijing believes that the city is in a critical situation. The previous disturbances and pandemic have dealt a heavy blow to Hong Kong’s economy. Now people are reviving these destructive acts when the pandemic has yet to end … the central government has to condemn and criticise that.”

A Hong Kong government spokesman said the central government had the constitutional right and responsibility to make sure the “one country, two systems” principle was fully implemented in the city.

He said as the liaison office handled Hong Kong affairs on behalf of the central government, it was “reasonable” and “its responsibility” to express comments and concerns on the implementation of the Basic Law and one country, two systems.