Nobody had cordoned off the square or moved onlookers out of the hail of glass.
The police quickly pushed people back as they cordoned off seven square blocks around the building and began evacuating hundreds of people from the area, which is home to Carnegie Hall as well and hundreds of residential apartments and a luxury hotel.
The BBC's Jim Muir in Cairo says the military, who have cordoned off the square with tanks, are very relaxed and letting people come and go.
Cleveland originally planned to cordon off 3.3 square miles (8.5 square km) around the convention site as an event zone where free speech mobility would be limited.
Streets around the Grand-Place, the city's neo-Classical central square, were cordoned off late Saturday night.
On June 25th police cordoned off the central square in Gori, the birthplace of Stalin, and, in the dead of night, toppled a statue of the former dictator that had survived the Soviet collapse and Russian bombardment in 2008.
Notable, is the variety of demonstration locations in Moscow (Belorussky station, Pushkin Square, Mayakovsky square, Red Square cordoned off) and throughout the country (Petersburg, Samara, Perm, Vladivostok, Tambov, Vologda, Ryazan, Krasnodar, Kazan as the list grows.
Police cordoned off Taksim, a square lined with hotels and restaurants frequented by tourists, while a member of the police bomb squad was seen opening what appeared to be a bag, the witness said.
In the non-virtual world the Kremlin has deployed its youth movement to occupy large public spaces, brought troops into the centre of Moscow and cordoned off the main squares.
Occasionally, their fiefdom is invaded by police sweeps, such as the highly publicized operation mounted May 3, when police suddenly set up wooden barricades and cordoned off a four-square-block area known among neighbors as "Drug Alley". On the first night of the operation, dozens of policemen stood guard by the barricades, brandishing night sticks.
Meanwhile, authorities in Congo are attempting to cordon off 125 square miles of rainforest.
