SHANGHAI, June 29 -- Strong demand for "the world's most expensive piece of iron" continued in Shanghai in June as an auction of car registration plates attracted record high number of bidders.

The number of bidders increased to over 135,600 this month, roughly 20,000 more than last month, but only 7,400 plates are available.

The lowest winning bid fell to 73,800 yuan (about 12,000 U.S. dollars), down 600 yuan from May.

The average price of a Shanghai license plate for private car owners also dropped to 73,896 yuan.

It was the first time that prices for Shanghai plates, which the city's Party chief Han Zheng once publicly called, half-jokingly, "the world's most expensive piece of iron", have dropped over the past few months.

Shanghai's municipal government had earlier announced a ceiling for first-round bids in 2014, set according to the lowest ceiling last year.

The move prompted a price drop from last year's record high of nearly 100,000 yuan in the smog-choked city where traffic jams are commonplace.