Even before dawn broke, some people tried to charge the gate, prompting a swift response from officials, witnesses said.
Police were also removing people from the queue for being too pushy and at least one man was detained.
Prospective buyers have been limited to two tickets at the same competition, but officials have promised the last tranche holds tickets for events at every venue, if not every event.
Many want tickets for an event in the National Stadium, dubbed the Bird's Nest, or the Water Cube, the two showpiece stadiums that have changed Beijing's landscape.
Image: Chinese policemen clash with journalists at the Olympic Green.
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