In London, trading was severely disrupted as panic spread through the financial district. The stock exchange said it understood that some firms would not be able to meet their trading obligations.
The FTSE-100 index was already suffering this week but it plunged to new depths soon after the attacks, with £67bn wiped off the value of the UK's leading companies.
The FTSE-100 was 287.7 points lower at 4746 when trading was suspended. It was the biggest ever one-day fall. The London market was still reeling as Building went to press.
Markets in Amsterdam, Frankfurt, Tokyo and Paris also fell sharply.
One construction analyst said: "This is terrible, terrible news and it will have repercussions for all sectors and stocks. Hang on tight."