NEW YORK, April 19 -- Metromedia Fiber Network Inc., said Tuesday it had defaulted on $975 million in debt held by an affiliate of Verizon Communications Inc. Metromedia, which builds high-speed networks in cities, said it was in default after failing to pay interest under a 30-day grace period on the debt. As a result, it said it may be forced to repay the debt under an accelerated schedule. The company is currently in discussions with the holders of substantially all of its indebtedness, including Verizon, about its restructuring.