Saying Tuesday's morning terrorist attack was a "horrible, vicious act" Mayor Rudolph Giuliani called on New York City residents to remain calm in the attack's aftermath.

Giuliani was joined by Governor George Pataki at an early-afternoon press conference, six hours after two hijacked planed crashed into the World Trade Center towers, causing their ultimate collapse.

The mayor said that more than 600 people have been treated for injuries at area hospitals. As additional 1,500 were treated in a makeshift area at Liberty State Park.

Giuliani said that 150 people were thought to be critically injured in the attack.

He said he did not know how many fatalities there are.

The mayor said that the National Guard has been deployed, and that his primary concern is keeping the citizenry safe.

"This is a tragedy that we thought would never happen," Giuliani said of the attack, which was accompanied by a similar attack on the Pentagon in Washington D.C.

"This is an attack on America and our way of life," added Pataki.