I was stunned by how quickly an act of mild protest had turned into a violent arrest, but it was not the only shock on Sunday. More than 20,000 people joined a rally against corruption, far more than anyone expected, the largest of dozens of protests across the country.

As fearsome-looking police in urban camouflage lined the streets, I was amazed at the festive attitude among the protesters. I saw teenagers challenging police with slogans like “You can't jail us all.” I saw a couple at the protest with their 1-year-old toddler as though enjoying a county fair. Instead of barbecue and games, however, there were the constant warnings of the police to disperse, or else.

The “or else” came any time someone tried to raise a sign or the yellow duck toy that has come to symbolize official corruption. Police would rip down the symbol or sign and haul in the person responsible for it. In the end, police would end up pushing protesters out of Pushkin Square, the center of the demonstration, and detain more than 1,000.

I managed to escape that, but I got pushed around a bit when I took my camera too close to a group of police officers as they were setting up a barricade. One of the officers grabbed me and stopped me from falling. He grinned and said, “You'll want to take off that lens cap.”

When police estimated that 7,000 people joined Sunday's rally in Moscow, they weren't counting the crowds streaming up and down a mile-long stretch of Tverskaya Street.

Thousands of protesters crowded into central Moscow's Pushkin Square to protest corruption in the upper ranks of government.

As the rally progressed and the crowd grew, riot police in full combat gear took up positions along Tverskaya Street, central Moscow's main artery. At first, the police were calm and even let people take their pictures.

The yellow duck is the symbol of official corruption. The toy refers to a lavish duck house at an estate allegedly owned by Russia's prime minister. Holding one up during the protest was enough to get you arrested at Sunday's rally in Moscow.

Seconds after the protester (center left, wearing a dark shirt) walked past riot police with a sign saying “We've found your money” and showing pictures of the property allegedly belonging to Prime Minister Dmitry Medvedev, a group of officers ran him down, threw him to the ground and dragged him into custody.

Undeterred by police standing a few feet away, these teens chanted, “You can't jail all of us!”