What was the main stock exchange whose collapse marked the Panic of 1873's immediate beginning?

The collapse of the Vienna Stock Exchange marked the immediate beginning of the Panic of 1873.

On May 9, 1873, known as Black Friday in Vienna, speculative share prices collapsed after a period of rapid expansion and heavy investment in railways, construction, and property. The exchange crash exposed weak businesses and excessive borrowing.

Financial stress spread from Austria-Hungary to Germany and then across the Atlantic. In the United States, the failure of the banking firm Jay Cooke & Company helped trigger a major panic in September 1873, followed by a long economic downturn.

The crisis is often called the Long Depression, although historians debate how that label should be applied and how long the depression lasted. It was not the same event as the 1929 Wall Street Crash.