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Hong Kong (CNN) -- Hong Kong police and pro-democracy protesters faced off Friday morning outside the office of the city's chief executive -- the latest sign of simmering tensions in the city, even after the announcement of potential breakthrough talks.

The demonstrators blocked two vehicles, which authorities say contained food meant for police, from entering the premises of Chief Executive C.Y. Leung's office building. They chanted "shame" and "we don't trust you."

At midnight Thursday, a deadline set by protest leaders for Hong Kong's leader to resign passed without major incident.

Several dozen police officers were at the scene Friday morning, carrying helmets, shields and, in a few cases, batons.

One of the protest leaders, Benny Tai, co-founder of the Occupy Central movement, told journalists late Friday morning that protesters should continue the policy of nonviolence and also be careful, as the police may be trying to provoke a reaction through their tactics.

He also said that, in light of Leung's refusal to step down, the pro-democracy movement will keep looking at the content and the process of the dialogue before deciding what to do.

CNN's Ivan Watson said that although protesters have also barricaded the entrance to Leung's office, they have allowed people in street clothes through. They appear to be office workers, Watson said, and were carrying what seemed to be official ID badges. The protesters also handed bundles of newspapers over the barricades.

Earlier in the day, a police officer on the scene collapsed and was taken out by ambulance. Protesters allowed the vehicle unimpeded passage. A police spokesperson said the officer was in serious condition.

As the working day begun, protesters chanted, "It's 9 a.m., where is he?" referring to Leung, and mockingly called the chief executive a "turtle" for refusing to show his face.

Meanwhile, a government news release announced the temporary closure of the Central Government Offices. Around 20 branches of various banks were also closed Friday.

The confrontation comes amid intense pressure by the student-led movement for Leung's resignation. Yet he has refused do so, even warning amid protesters' threats to occupy government buildings that there will be "serious consequences" if police lines are crossed.

The chief executive did make one move demonstrators have welcomed, albeit cautiously: He said his second in command, Chief Secretary Carrie Lam, will meet representatives of the Hong Kong Federation of Students to talk "constitutional development matters."

Until now, the two sides have talked past each other in the streets, in public pronouncements and through the media, not directly to one another. Besides Leung's ouster, protesters are demanding a reversal of a recently enacted policy giving Beijing veto power on who can be chief executive candidates in the 2017 election, a move that they say shows the central government is encroaching too much on Hong Kong's affairs.

The student federation confirmed the Lam meeting -- even if the time and exact agenda haven't been announced -- expressing "hope (Lam) will be committed to this city and willing to have an open dialogue." In addition to a swipe at Leung as someone who "has already lost his integrity and ruling legitimacy," the group said "the main focus of the debate should be ... political reform."

Talks or not, the Hong Kong Federation of Students also stressed that its protesters wouldn't leave the streets anytime soon.

"Hong Kong people shall continue its occupying movement until genuine universal suffrage comes to light," the group said, urging supporters to "persevere their occupying action ... and be on guard at all times."

"... Whether we will bring our actions to the next level, will very much depend on the outcome of the dialogue."

Many demonstrators weren't impressed by Leung's remarks. One man, identified only as Ray, dismissed them as "rubbish."

Another, who went by Harry, accused Leung of trying to pass the buck without making a bold decision that might irk Beijing like scrapping the policy allowing the central government to vet candidates.

"C.Y. is just putting the ball in the citizens' court again," he said.

Authorities may be getting impatient

A government statement said the gathering protesters were paralyzing traffic in the area and warned them not to charge government buildings or police cordons.

The pro-democracy activists still occupy crucial parts of the Asian financial hub. But patience may be wearing thin as the Hong Kong government reopened Friday after two public holidays, as protests have extended into a fifth day.

All schools in the Central and Western districts were closed Friday. The government said the protests have "increasingly serious impacts on people's livelihood, Hong Kong's economy and even government operations."

Hardly relenting, protest leaders have urged more supporters to come out -- claiming that police are gathering supplies, including tear gas and pepper spray, to clear them out.

How tear gas united Hong Kongers

"The police are sending gear to the (chief executive's) office," protest leader Robert Chow said. "Please go over there to offer support. We are fighting for universal suffrage and the right to nominate our leaders."

A CNN journalist saw containers being brought into the government offices Thursday evening labeled "round, 38 mm rubber baton," or rubber bullets.

Hong Kong police would not confirm the contents of the containers. Other protest leaders urged unity among a group that is rapidly becoming fatigued after several nights on the streets.

"The volunteers here are working so hard to try and keep everyone united," said Benny Tai, co-founder of the Occupy Central movement. "I really hope that we can continue to work together to continue to stand firm, to continue fighting so we can have true democracy."

A high-ranking Chinese official has denounced the protests as "illegal acts" and reiterated China's view that what happens in the special administrative region is purely a domestic matter.

"Hong Kong affairs are China's internal affairs," said Wang Yi, the Chinese foreign minister.

Hong Kong protests: What's next?

Wang is the highest-level official to speak critically about the Hong Kong pro-democracy demonstrations. Wang made the comments while standing next to U.S. Secretary of State John Kerry, who has expressed support for universal suffrage.

Can the protests last?

On Thursday, crowds at the main protest site appeared noticeably thinner. Some said the turnout could have been smaller because it was a traditional holiday when families visit their ancestors' graves.

Fewer people were camped out and sitting on the roads than earlier this week. More people were milling about and wandering the streets.

The protests have snarled traffic in Hong Kong's main finance and commerce districts. It remains unclear how long the protests can maintain support and continue to draw the numbers that have so far clogged main arteries.

"If (the students) drag this on for a really long time, they're going to start losing some of their support," said David Zweig, a professor at the Hong Kong University of Science and Technology, as he stood at the main protest site.

At the same time, there is "enormous pressure" for the students to gain something from the mass demonstrations.

Tensions rose after a late August decision that gave Beijing control over the slate of Hong Kong chief executive candidates in 2017. It will let the city's 5 million eligible voters pick a winner, rather than a 1,200-member committee stacked with Beijing loyalists that has chosen past leaders. But critics argue that the right to vote is pointless if the candidates are handpicked by Beijing.

Meet the 17-year-old student agitating for change in Hong Kong

They launched the large-scale civil disobedience, which was dubbed the Umbrella Revolution after umbrellas became symbols of the movement when they were used to shield against police tear gas and pepper spray on Sunday.

The huge crowds demand true universal suffrage without restrictions on candidates. To date, 94 people have been injured in the protests, according to a government media officer. Despite the huge turnout on the streets, not everyone in Hong Kong is behind the protest movement.

Leung has backing from pro-Beijing groups such as the Silent Majority for Hong Kong, which has had its own rallies. They argue that pro-democracy activists will "endanger Hong Kong" and create chaos.

CNN's Elizabeth Joseph, Esther Pang, Anjali Tsui, Jethro Mullen, Greg Botelho and Laura Smith-Spark contributed to this report.