(Adds more details; Hong Kong market performance)
SHANGHAI, Sept 16 (Reuters) - China stocks posted their biggest gains in three weeks on Wednesday, helped by a spike in late trade, a phenomenon which many traders believe is a sign of government intervention to push up prices before the closing bell.
The country's main share indexes rose 5 percent, erasing much of the sharp losses seen on Monday and Tuesday which were attributed to investors' concerns about the slowing economy. Stocks had finished the morning session only marginally higher.
Late rallies have become more frequent in China's markets since the government launched a massive rescue package this summer to avert a full-blown market crash.
The blue-chip CSI300 index ended up 157 points at 3,309.25, while the Shanghai Composite Index closed up 148 points at 3,152.26, but are still down about 40 percent since mid-June.
Volumes, however, remained subdued, suggesting there was little investor conviction behind the move. The 10-day average volume in Shanghai has been sliding steadily since August.
"Volume has been tiny..., so you don't need a lot of money to push up prices," said David Dai, Shanghai-based investment director at Nanhai Fund Management Co.
Dai said many buyers on Wednesday, including some hedge funds and individuals, were unlikely to hold stocks long, fearing further price rises would trigger fresh waves of selling.
"It's a technical rebound. We're likely to see more volatility ahead."
Mainland shares rose across the board, with over 1,000 stocks hitting their 10 percent daily upward limit.
Even CITIC Securities, whose shares were walloped in morning trading by news of a probe into executive insider trading, reversed losses and ended the day up 7 percent.
On Tuesday, CITIC Securities said police were investigating senior managers including general manager Cheng Boming, who were suspected of insider trading and leaking information.
Small-caps, which fell more sharply than blue-chips in the previous two days, bounced in the same ferocious manner, with the growth board ChiNext jumping 7 percent, while a key index tracking tech shares surging 9.4 percent.
Gains in Chinese shares gave a boost to Hong Kong stocks, with the benchmark Hang Seng index ending the day up 2.4 percent.
Reporting by Pete Sweeney and Samuel Shen; Editing by Shri Navaratnam & Kim Coghill