A heavily armed, militarised police force fired teargas and rubber bullets to force hundreds of protesters out of the centre of a small Missouri town on Wednesday, as a crackdown on demonstrations over the killing of an unarmed 18-year-old intensified.

Dozens of officers, some carrying assault rifles, advanced with a pair of armoured trucks on the young and predominantly African American crowd in Fergsuon, after two glass bottles were thrown at their lines from a largely peaceful protest against the shooting of Michael Brown, who was black, by an officer from the city's overwhelmingly white police department.

For hours, police snipers trained their weapons on demonstrators who protested with their hands up as an emblem of peaceful protest. When events escalated on a fourth night of tension in this city of just 21,000 people, protesters described being subjected to military-style tactics as they fled through gas-filled residential side-streets.

Demonstrators raise their hands while protesting against the killing of teenager Michael Brown, in Ferguson, Missouri, on Wednesday. Photograph: Mario Anzuoni/Reuters

Police clearing the main drag of Ferguson, a suburb of St Louis, also arrested two reporters, including one from the Washington Post. Marty Baron, its editor, condemned the arrest as "an assault on the freedom of the press to cover the news". The Huffington Post, whose reporter was also detained, said the journalists were subjected to "militant aggression" and treated as "enemy combatants". A camera crew from al-Jazeera America said they were shot by rubber bullets.

The White House said early on Thursday that Barack Obama had interrupted his vacation on Martha's Vineyard, Massachusetts, to be briefed by the US attorney general, Eric Holder, on the escalating clashes between police and demonstrators.

The night's events were described as "deeply troubling" by Jay Nixon, Missouri's governor, who will visit the area on Thursday. "While we all respect the solemn responsibility of our law enforcement officers to protect the public, we must also safeguard the rights of Missourians to peaceably assemble and the rights of the press to report on matters of public concern," he said.

Protesters were angry at the shooting of Brown on Saturday afternoon by a Ferguson city police officer, and at the response by authorities. Police have said the shooting followed an assault on the officer. But a friend who was walking with Brown said the 18-year-old was grabbed after declining a request to get on to the pavement, and then shot repeatedly as he tried to run away. The officer has still not been named. St Louis County police are investigating.

A riot police officer aims his weapon during demonstrations over the shooting of Michael Brown. Photograph: Mario Anzuoni/Reuters

After standing guard as demonstrators gathered through the afternoon on Wednesday, the police began clearing West Florissant Avenue, Ferguson's main street, as the evening drew in. Just before 7pm, they evicted customers from a branch of McDonald's towards the eastern end of the street.

Inside, working, were reporters Wesley Lowery of the Washington Post and Ryan Reilly of the Huffington Post. In statements, their publications said the pair were videoing and photographing the police actions and were then arrested and handcuffed as they tried to leave. The reporters were eventually released after spending about half an hour in a cell.

"Police militarisation has been among the most consequential and unnoticed developments of our time, and it is now beginning to affect press freedom," said Ryan Grim, the Huffington Post's Washington bureau chief.

At least 10 people were arrested in all, according to police, including Antonio French, a St Louis city alderman who had been observing the protests and covering them on social media. French was reportedly charged with unlawful assembly.

Riot police stand guard at the protests. Photograph: Mario Anzuoni/Reuters

At 8.40pm, at the other end of West Florissant, two bottles smashing prompted a furious reaction from the police line. Protesters were warned over a Tannoy to leave the area immediately or be arrested. "Your peaceful protest is no longer peaceful," the police voice said.

When the protesters refused to retreat, a high-pitched siren blared from a police Swat truck and several canisters of teargas were fired towards the crowd in quick succession. "Return to your vehicles, return to your homes," the police Tannoy said. Yet protesters regrouped, linked arms and continued to chant at the police, some screaming obscenities.

At 9.08pm, a barrage of gas canisters and flashbangs were shot at the crowds, prompting chaos. Many protesters ran south down Lang Drive. But, even as they fled, the police line continued to fire gas canisters and crackers in their direction. No one could be seen throwing anything at the officers.

Some canisters landed and emitted plumes of smoke in the front gardens of residents as they sat inside their homes. Others hit cars and trucks as people tried to escape.

People in Ferguson protest against the death of teenager Michael Brown. Photograph: Mario Anzuoni/Reuters

"Our property value has gone down, our taxes have gone up and our insurance has gone up because St Louis County came out here shooting at these kids while they were sitting in the street," said one woman living on Lang Drive, who declined to be named. "They started this."

Rubber bullets were fired at people who remained on West Florissant. One young man, who had his arms aloft in the "hands up – don't shoot" gesture that has come to define these protests, was shot several times. After he fell to the ground, he was pounced on by several officers.

"They hit me on the hand with a teargas bomb," said Tony Giegers, 27. "It exploded on me kind of like buckshot. It sprayed everywhere. It hit me there on my hand and burned my hand. It's burning right now, it's burning up." His friend DeJuan, 19, said he was struck in the right leg.

A highly visible camera crew for al-Jazeera America reporting on the clashes said that they, too, were shot at with rubber bullets. The Guardian watched as a gas canister was fired directly at them as they attempted to record a piece to camera. The crew and their presenter fled.

"The police are responding with force, which you can understand to a certain degree," said Justin, a 29-year-old protester. "But this amount of force was unwarranted. This is an abuse of force. No one is firing guns at them or running at them. This is an overreach.

Michael Brown, who was shot and killed by police in Ferguson on Saturday. Photograph: Anonymous/AP

"Were people yelling at them? Were people shouting obscenities? Yes, that's normal. But no one rioted, no one looted, no one threatened to harm anyone. This is getting out of control. It's like a military action."

"They militarised the police in this small county in Missouri," said a man who gave his name as Kevvy. "They got military trucks and people coming in from all over the state just for a protest at something they did."

Brian Schellman, a spokesman for St Louis County police, told the Guardian at the law enforcement command centre late on Wednesday that chiefs would review tactics after the night's events. "Our commanders are definitely going to review and see if we can do something better," he said. However Schellman said that the officers involved "acted with great restraint". "Bottles were being thrown at them; we had molotov cocktails being thrown; we had a police officer break an ankle from a thrown brick. So when the assaults start coming on police officers that's the measures that are taken. We don't wanna have to use gas on anybody."

Police at the demonstration in Ferguson. Photograph: Scott Olson/Getty Images

Schellman said he could not say whether any bottles struck officers. However, he said, "the fear of threat like that is still construed as an assault. Injury does not have to happen for an assault to happen."

He said that police did not yet know whether they would repeat the tactics on Thursday if protesters turned out again, defying a request from the city to demonstrate only in daylight hours.

Some said they would. "We got to retaliate to the force they are using," said Kevvy. "If they try and hurt us we try and hurt them. That's just the way it is. People that live in this community, people out here for Mike Brown, people that's black."