Beijing's already rapid pace of construction for the 2008 Olympics is shifting into higher gear, with more workers being hired to stay on schedule.

This year's goal is to complete the main structures at all venues so construction can be finished as planned by the end of 2007, Jin Yan, deputy director of the Beijing office in charge of Olympic projects, said at a news conference Tuesday.

"This is a crucial year," Jin said.

Construction officials said work has begun on 20 of the 31 venues, and the number of workers will rise from 17,000 to more than 30,000 in the middle of this year as more projects get under way.

Beijing has been criticized for a seemingly frenetic pace of Olympic construction. Only 10 months ago, the International Olympic Committee urged Beijing to slow down so venues will not be completed too early and run up additional operating expenses.

But Jin and other officials defended the pace given the complexity of staging the Olympics.

"The details will decide whether we will succeed or fail, and now we're getting down into the details," Jin said.

At the National Stadium, the city's signature Olympic project, more than 2,000 workers on Tuesday were assembling arched steel towers - some nearly 230 feet tall - for the building's exterior, whose shape and latticed frame resemble a bird's nest.

Some of the larger showcase venues are using designs and materials that are cutting-edge for China, presenting technical problems that require more time for construction, the officials said.

At the 91,000-seat National Stadium, for example, workers using detailed computer-generated blueprints have to assemble the massive steel towers on site, said Zhang Hengli, a deputy general manager with the state company that owns the stadium.

Next door, at the National Aquatics Center, workers are laying a framework of steel pentagons and hexagons over which a futuristic translucent material will be stretched.