UPDATE 4-Hundreds return to Hong Kong streets as metro, shops shut after violence

* Protest marchers wear face masks in defiance of ban

* Transport paralysed in Hong Kong after night of chaos

* "Extreme violence" justified use of emergency law, Lam says

* Police shot teenager, as businesses, metro stations torched

* More protests planned through weekend (Adds details on protests, shop shutdowns, Vans controversy)

By Felix Tam and Greg Torode

HONG KONG, Oct 5 (Reuters) - Hong Kong’s metro system stayed shut on Saturday, paralysing transport in the Asian financial hub, and malls and shops closed early after a night of chaos in which police shot a teenage boy and protesters torched businesses and metro stations.

Friday's protests across the Chinese-ruled city erupted hours after its embattled leader, Carrie Lam, invoked colonial-era emergency powers for the first time in more than 50 years to ban the face masks demonstrators use to hide their identities.

The night's "extreme violence" justified the use of the emergency law, Beijing-backed Lam said in a television address on Saturday.

"The radical behavior of rioters took Hong Kong through a very dark night, leaving society today half-paralyzed," she said in pre-recorded remarks.

"The extreme violence clearly illustrated that Hong Kong's public safety is widely endangered. That's the concrete reason that we had to invoke emergency law yesterday to introduce the anti-mask law."

But undeterred by the ban and transport shutdown, several hundred pro-democracy protesters, many wearing masks, took to the streets on Saturday, marching through the normally bustling central district of Causeway Bay.

Other groups gathered in Sheung Shui and Tsim Sha Tsui districts as the sun began to set.

"We’re not sure what is going to happen later, but we felt we had to get out and show our basic right to wear a mask," said one protester, Sue, 22, who wore a black mask and dark glasses to the Causeway Bay march.

"The government needs to learn it can’t squeeze Hong Kong people like this."

The increasingly violent demonstrations that have roiled the city for four months began in opposition to a bill introduced in April that would have allowed extradition to mainland China, but they have since spiraled into a broader pro-democracy movement.

The unrest has plunged Hong Kong into its biggest political crisis since its handover from Britain to China in 1997 under a “one country, two systems” formula that granted it autonomy and broad freedoms not enjoyed on the mainland.

China's Hong Kong and Macao Affairs Office said on Friday the protests were evolving into a revolution backed by foreign forces and could not continue indefinitely.