To the delight of onlookers and the dismay of architectural critics, a massive metal eye has risen from the surface of the Thames River and taken up position staring down at Big Ben.

It's known as the London Eye, but the gleaming orb that rose from a barge this weekend on the south bank is actually the world's largest Ferris wheel, erected to mark the millennium. At the top, riders will be 450 feet above the river. That's about four-fifths the height of the Washington Monument, but twice as tall as Big Ben and the other London landmarks sharing the central stretch of the river near Westminster Bridge.

The view from that bridge--the visage that prompted Wordsworth's famous apostrophe, "Earth has not anything to show more fair"--includes some of the city's most beloved structures, including Big Ben, the Houses of Parliament, County Hall and Westminster Abbey. The juxtaposition of a postmodern metal wheel with those Gothic and neo-gothic treasures has prompted some gnashing of teeth among art critics.

But the two London architects who dreamed up the project, Julia Barfield and David Marks, say they've received strong public support. More importantly, they raised the $32 million construction budget--largely from German and Japanese investors--and then a got a major corporate name here, British Airways, to sign on as chief sponsor. It was the airline that came up with the appellation London Eye.

The "observation wheel"--it's technically not a "Ferris" wheel, because it is supported on only one side--is scheduled to spin for five years. But it will probably last much longer if it proves a hit with locals and tourists. A one-rotation ride will cost $12.60 ($4 less for children) and last about 30 minutes. The wheel is to spin so slowly that it won't stop while passengers step on and off.

One flaw with this massive millennium monument is that it won't open to the public in time for this New Year's Eve. A series of construction delays, including one embarrassing moment last month when the chairman of British Airways pushed the ceremonial button to lift the wheel, only to have the cables snap, has put the Eye behind schedule. Planners now say they hope to be spinning early in 2000.

CAPTION: The massive wheel overlooks Parliament and other London landmarks.