By NBC News’ Nancy Smith

LONDON – I had intended to do a little Christmas shopping on the way home from work Thursday, but instead walked straight into a riot at Oxford Circus. 

Traffic all over London had been at a virtual standstill all afternoon, the result of road closures due to the massive student demonstrations, so I had headed out on foot toward Oxford Street. 

Nothing seemed unusual – stores were open, Christmas decorations were sparkling and people were moving in and out of stores with their purchases. 

One moment I was surrounded by Christmas shoppers, the next I heard a groundswell of chanting, bricks and stones were flying overhead, and garbage cans were crashing onto the ground. The peaceful holiday landscape was suddenly transformed into a scene of urban violence. 

The swelling crowd and the rising chants seemed to come from nowhere, catching shoppers and bystanders unaware, the noise and the violence quickly escalating.

The glass on two of the front doors of Topshop’s flagship store at Oxford Circus were shattered, the doors closed and a line of security men formed inside the shop. Pedestrians fled as debris thrown by the demonstrators began to crash land nearby.

A forlorn student, holding his banner to protest against the rise in student fees but standing away from the mayhem, seemed mystified and frightened by the violent turn the demonstration had taken.

Carl De Souza / AFP - Getty Images

British riot come under attack from flares as they clash with protestors during student demonstrations in Parliament Square, in London, on Thursday.

Slideshow: London student protests

As rocks and stones flew overhead, an armored police vehicle slowly made its way through the melee. A line of police officers, carrying shields and wearing protective visors over their faces, began to form a barrier between the increasingly belligerent protestors and the bewildered shoppers. 

Although most of the pedestrians retreated back down Oxford Street or sought safety behind an abandoned bus, a significant number walked toward the police line, snapping pictures with their mobile phones. The flash of the phones created an eerily dazzling effect combined with the illuminated seasonal decorations suspended above the mayhem. 

With police quickly blocking access into nearby side streets, the only escape route was back down Oxford Street, where Christmas shopping resumed as normal. Only minutes away from the confrontation, the calm inside the stores was interrupted only by the squawk of walkie-talkies advising security guards of the status of the demonstration.  Shoppers went on about their business.

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I suppose I should have known it was not the time to venture out for a shopping trip. Earlier in the day, on the way to an appointment at Parliament Square, near the Houses of Parliament and Big Ben, I had seen an enormous police operation preparing for the anticipated student demonstration. It looked like a police state. Barricades had been set up, spare barriers were stacked along the square and police were suiting up for potentially violent confrontations. 

By the time I left the area, lines of police were beginning to form at the entrance to the square.  Most of the police at that point in the early afternoon were answering questions from tourists, fearful that the overwhelming police presence signified something sinister. Roadblocks were already causing traffic chaos and it took twice as long as usual to get back to the office.

Later in the afternoon, thousands of students joined the demonstration, Parliament passed the bill raising student fees and breakaway demonstrations spread across town, including the violent encounter I witnessed at Oxford Circus.