Five days after being rocked by the bankruptcy of Lehman Brothers, trading on Wall Street surged Friday as the Dow rose 370 points on the news that the federal government is planning to buy up to a half-trillion dollars of banks' mortgage-related debts to help stem the financial crisis.

Calling the plan an "unprecedented action," President George Bush addressed the nation from the White House Rose Garden and said the plan would need the cooperation of Democrats and Republicans.

"We believe that this decisive government action is needed to preserve America's financial system and sustain America's overall economy," Bush said. "These measures will require us to put a significant amount of taxpayers' dollars on the line. This action does entail risk. We expect that this money will eventually be paid back."

The specifics of the plan are still being worked out, but it would be the biggest bailout in U.S. history and the greatest government intervention in financial markets since the Great Depression.

News of the plan drew mixed reaction on Wall Street, with some in favor and others skeptical.

"I think its a really great thing," said one financial employee. "That's why we have the Fed Reserve and there are certain circumstances where it's necessary, so I thinks it's a good thing."

"I personally think that you have these high powered executives that are responsible for running these companies and when they fail in their jobs the government is stepping in and using tax dollars to bail them out," said another. "I believe these companies should be on their own."

Amid the market crisis, the Securities and Exchange Commission has temporarily banned short selling in financial firms.

Short selling is when traders try to make a profit by selling borrowed shares of stock in the hopes that the prices will drop. Traders then buy the stock at the lower price and pocket the difference.

Market experts say allowing short selling contributes to the efficiency of the market, but critics say it can contribute to a company's downfall.

New York Attorney General Andrew Cuomo has launched an investigation into short selling of shares of companies including Lehman Brothers, AIG, Morgan Stanley and Goldman Sachs.

Cuomo agreed that there's nothing illegal about short selling on its own, but said he suspected some traders may have spread false information to try to cash in.

Meanwhile Treasury Secretary Henry Paulson said Friday that the proposed federal bailout could require hundreds of billions of dollars. He and Federal Reserve Chairman Ben Bernanke met with congressional leaders Thursday to reach a consensus on legislation that will be presented before Congress adjourns next week.

"To restore confidence in our markets and our financial institutions so they can fuel continued growth and prosperity, we must address the underlying problem," said Paulson. "The federal government must implement a program to remove these illiquid assets that are weighing down our financial institutions and threatening our economy."

Paulson said there is too much at stake for the federal government to just sit by.

"The financial security of all Americans, their retirement savings, their home values, their ability to borrow for college and opportunities for more and higher-paying jobs depends on our ability to restore our financial institutions to sound footing," said Paulson.

The Bush administration also authorized the treasury to tap up to $50 billion from a Depression-era fund to protect the holdings of eligible money market mutual funds.

Some Democrats in Congress said they're skeptical about the White House's economic rescue plan.

Senate Banking Committee Chairman Chris Dodd demanded more details on the costly plan and said his panel had not yet heard any specifics.

The bipartisan committee has vowed to work across the aisle to take on the fiscal crisis.