TORONTO - How can a city so empty be so on edge?

Canada's biggest metropolis was a contradictory mix of sedate traffic, deserted boulevards and frayed nerves Thursday as Toronto sat poised on the knife-edge of this weekend's G20 summit.

"It looks like a movie set," observed a bored busboy, gazing across a near-empty, midday luncheon patio down a deserted four-lane street.

If so, the script would have to be post-apocalyptic. Or is that pre-apocalyptic?

Jumpy Torontonians can't seem to make up their minds.

In a city beseiged by security forces, where posses of cycling cops pedal empty backstreets, arguably the safest downtown core in the country is wracked by paranoia.

"The G20 won't shut us down!" read a defiant chalkboard sign outside a sandwich shop Thursday.

Across the street, the windows of an office tower and condo unit were boarded up.

A man driving a dilapidated car with a cross-bow and chainsaw aboard -- standard issue in some parts of rural and northern Ontario -- caused the closure of a downtown city block and a security alert Thursday.

A day earlier, the first gut reaction of many locals to a moderate, if highly unusual, earthquake, was to think the rumbling signalled some kind of summit-related terrorist attack.

The cybersphere caused a minor sensation when someone used Twitter to relay rumour of a machine-gun toting madman at the Eaton Centre. The landmark shopping mecca had been evacuated, others breathlessly chimed in, and a manhunt was underway.

Never mind, police soon tweeted. No such incident took place.

But an unattended suitcase did shut down the subway for 45 minutes at rush hour.

Police are warning business people not to wear suits downtown, lest they attract unwanted protester attention.

"Anyone with any sense had already left town," Bob Dylan once sang to foreshadow a looming drama.

With tales of workers stuck in the city for the G8-G20 sending their young families down the highway for the weekend, it could be the summit mantra.

And yet the physical environs of downtown Toronto are preternaturally calm.

Lake Ontario, this windy summer day, was devoid of pleasure craft.

The raised Gardiner Expressway as seen from the air, usually a still-water river of automotive steel, was dotted with single vehicles like lazy floating rafts.

"Take a seat wherever you want," the busboy told customers, waving to his empty restaurant patio.

And near the corner of Yonge Street and Wellington, in the very heart of deepest downtown Toronto, groups of two and three cars eased past at half-block intervals during the mid-afternoon lull.

"This is weird, man," a pony-tailed worker, sauntering across the empty Yonge at mid-block, called to his friends on the other side.

"You could get a road hockey game going!"