Police in Hong Kong are warning of possible violence on the eve of another mass protest against a controversial extradition bill after making “the largest seizure” of explosives in the territory.

The seizure came on the day that thousands of pro-government supporters took part in a rally.

Millions in the former British colony have thronged the streets since 9 June to protest against a proposed law allowing for the extradition of individuals to mainland China, where courts are controlled by the Communist party.

Many in Hong Kong believe Sunday’s protest, which has been sanctioned by police, will end in violent conflicts, as in previous weeks.

Quick guide What are the Hong Kong protests about? Show Hide Why are people protesting? The protests were triggered by a controversial bill that would have allowed extraditions to mainland China, where the Communist party controls the courts, but have since evolved into a broader pro-democracy movement. Public anger – fuelled by the aggressive tactics used by the police against demonstrators – has collided with years of frustration over worsening inequality and the cost of living in one of the world's most expensive, densely populated cities. The protest movement was given fresh impetus on 21 July when gangs of men attacked protesters and commuters at a mass transit station – while authorities seemingly did little to intervene. Underlying the movement is a push for full democracy in the city, whose leader is chosen by a committee dominated by a pro-Beijing establishment rather than by direct elections.

Protesters have vowed to keep their movement going until their core demands are met, such as the resignation of the city’s leader, Carrie Lam, an independent inquiry into police tactics, an amnesty for those arrested and a permanent withdrawal of the bill. Lam announced on 4 September that she was withdrawing the bill. Why were people so angry about the extradition bill? Beijing’s influence over Hong Kong has grown in recent years, as activists have been jailed and pro-democracy lawmakers disqualified from running or holding office. Independent booksellers have disappeared from the city, before reappearing in mainland China facing charges. Under the terms of the agreement by which the former British colony was returned to Chinese control in 1997, the semi-autonomous region was meant to maintain a “high degree of autonomy” through an independent judiciary, a free press and an open market economy, a framework known as “one country, two systems”. The extradition bill was seen as an attempt to undermine this and to give Beijing the ability to try pro-democracy activists under the judicial system of the mainland. How have the authorities responded? Beijing has issued increasingly shrill condemnations but has left it to the city's semi-autonomous government to deal with the situation. Meanwhile police have violently clashed directly with protesters, repeatedly firing teargas and rubber bullets. Beijing has ramped up its accusations that foreign countries are “fanning the fire” of unrest in the city. China’s top diplomat Yang Jiechi has ordered the US to “immediately stop interfering in Hong Kong affairs in any form”. Lily Kuo and Verna Yu in Hong Kong

During the past two weekends, after tens of thousands of anti-extradition protesters participated in peaceful, sanctioned marches, hundreds have been locked into tense, hours-long standoffs with the police, who then used pepper spray and truncheons on protesters late at night while they tried to leave, arresting dozens and resulting in injuries among many protesters and police officers.

Hong Kong police said in a news briefing on Saturday that they had arrested a 27-year-old man on suspicion of possessing explosives, after finding a cache of explosives and weapons in an out-of-town industrial building. Supt Alick McWhirter, of the explosive ordnance disposal unit, said police on Friday night found around 2kg of high explosives, triacetone triperoxide, or TATP, which was “extremely powerful” and unstable.

“I think without a doubt, this is the largest seizure we have ever come across in Hong Kong,” he said.

Steve Li, senior superintendent of the organised crime and triad bureau, said police also found petrol bombs, corrosive acid, knives as well as metal rods on the premises. Police said they also found anti-extradition bill banners and clothes emblazoned with “Hong Kong National Front”, a pro-independence group.

The group’s spokesperson, Baggio Leung, told Hong Kong Free Press that he was not able to confirm why explosives were found there, and would only know more once the suspect was released on bail.

In a separate press briefing, police used the discovery of explosives to justify why they have barred the Sunday protest from finishing in Central, the heart of Hong Kong’s business district, as the organisers, the Civil Human Rights Front, had planned.

Police instead mandated that the protest should end in Wanchai, before the march reaches Admiralty, where the government headquarters is located, and Central. The move has angered many protesters but the police defended the decision by claiming that some planned to protest away from the approved route and threatened to use weapons.

Quick guide Democracy under fire in Hong Kong since 1997 Show Hide Hong Kong’s democratic struggles since 1997 1 July 1997: Hong Kong, previously a British colony, is returned to China under the framework of “one country, two systems”. The “Basic Law” constitution guarantees to protect, for the next 50 years, the democratic institutions that make Hong Kong distinct from Communist-ruled mainland China. 2003: Hong Kong’s leaders introduce legislation that would forbid acts of treason and subversion against the Chinese government. The bill resembles laws used to charge dissidents on the mainland. An estimated half a million people turn out to protest against the bill. As a result of the backlash, further action on the proposal is halted. 2007: The Basic Law stated that the ultimate aim was for Hong Kong’s voters to achieve a complete democracy, but China decides in 2007 that universal suffrage in elections for the chief executive cannot be implemented until 2017. Some lawmakers are chosen by business and trade groups, while others are elected by vote. In a bid to accelerate a decision on universal suffrage, five lawmakers resign. But this act is followed by the adoption of the Beijing-backed electoral changes, which expand the chief executive’s selection committee and add more seats for lawmakers elected by direct vote. The legislation divides Hong Kong's pro-democracy camp, as some support the reforms while others say they will only delay full democracy while reinforcing a structure that favors Beijing. 2014: The Chinese government introduces a bill allowing Hong Kong residents to vote for their leader in 2017, but with one major caveat: the candidates must be approved by Beijing. Pro-democracy lawmakers are incensed by the bill, which they call an example of “fake universal suffrage” and “fake democracy”. The move triggers a massive protest as crowds occupy some of Hong Kong’s most crowded districts for 70 days. In June 2015, Hong Kong legislators formally reject the bill, and electoral reform stalls. The current chief executive, Carrie Lam, widely seen as the Chinese Communist party’s favoured candidate, is hand-picked in 2017 by a 1,200-person committee dominated by pro-Beijing elites. 2019: Lam pushes amendments to extradition laws that would allow people to be sent to mainland China to face charges. The proposed legislation triggers a huge protest, with organisers putting the turnout at 1 million, and a standoff that forces the legislature to postpone debate on the bills. After weeks of protest, often meeting with violent reprisals from the Hong Kong police, Lam announced that she would withdraw the bill. Photograph: Dale de la Rey/AFP

“In recent days, after the end of protests … some used the opportunity to create chaos, this includes blocking roads, charging police cordon lines and even damaging public properties and attacking police officers,” Ch Supt John Tse, of the police public relations branch, said, adding that protesters had used metal rods and bricks to damage public properties and attack police.

“These make us anxious about the situation tomorrow,” he said.

Play Video 0:40 Police use pepper spray to disperse peaceful protesters in Hong Kong - video

Violent conflicts erupted after peaceful protests in out-of-town districts Sheung Shui and Sha Tin last Saturday and Sunday.

Police used truncheons and pepper spray against protesters, who threw objects such as umbrellas and plastic water bottles at them. After being chased by police into a shopping centre in Sha Tin late on Sunday, several protesters were also seen beating a police officer. More than 40 arrests were made.

Meanwhile, thousands of government and police supporters – many from pro-China groups – took part in a rally in Admiralty near the government headquarters.

“Hong Kong’s prosperity relies on stability and economic success,” said a businessman on stage. “Say no to violence!”