By 1930 BST it looked rather like a repeat of last year's May Day protest, with police holding a firm line and stopping any dispersal of the crowd.
Hold protestors captive, stop them from going home, and they'll get angry, someone will throw something, and police and media can justify their predictions and budget.
Bit less of a mess than last year, at least.
http://news.bbc.co.uk/hi/english/uk/newsid_1962000/1962981.stm