SHANGHAI • China's bike-sharing bubble is showing signs of strain, after the country's third largest firm said it was going under.

The country has seen a wave of iconic, brightly coloured shared bicycles hit its city streets over the last year, helping revolutionise urban travel but also drawing some public ire over mountains of bike clogging up sidewalks.

The chief executive of Bluegogo, Mr Li Gang, said in a letter last week that he had "made mistakes". He apologised to investors, partners and 20 million registered users of the firm's 830,000 blue bikes. "I was filled with arrogance," he admitted, while Reuters reported him as saying financing in the market had become a "bubble".

China's bike-sharing craze has been driven by huge investments, especially in the two market leaders Mobike and Ofo, which have raised billions of dollars from tech giants like Tencent Holdings and Alibaba Group Holding.

Following Bluegogo's collapse, users have complained of difficulty getting refunds of their deposits. Users of another bike-share company, Coolqi, complained of similar problems after the company went bust this month, Xinhua news agency reported on Thursday.

Meanwhile, photographer Chen Zixiang has captured images of a huge bicycle graveyard in the south-eastern city of Xiamen, reported The Guardian.

The towering pile had thousands of bikes from Mobike, Ofo and Bluegogo. It appears to be one of the largest amalgamations of discarded bicycles, the report said.