Reflecting concerns about the health of the global financial system, the Federal Reserve and the world’s other major central banks significantly escalated their assistance to global markets on Thursday, making almost $200 billion available after bank lending came to a near halt and threatened the global economy.

In a statement released at 3 a.m. in Washington, just as the markets opened in Europe, the Fed said that it had authorized a $180 billion expansion of its temporary reciprocal currency arrangements, known as swap lines, to allow banks to borrow more dollars in money markets at a lower rates.

Paul Mortimer-Lee, head of market economics in the London office at BNP Paribas, said the move reflected concerns that the financial markets now appeared to be facing their gravest problems since the Depression.

“We’re high on a mountain, with a thin rope and holding on by our fingertips,” he said. “Are policy makers scared? They should be.”