* HSI +0.6 pct, H-shares +0.5 pct, CSI300 +0.3 pct
* China Q1 growth at lowest in 18 months within expectations
* Mainland securities firms see upswing
BEIJING, April 16 (Reuters) - Chinese and Hong Kong investors shrugged off China's first quarter growth that was the slowest in 18 months, leaving all major indices in positive territory on Wednesday.
On the mainland some of the strongest gains came from a rebound in securities firms, a sector that has proved volatile after last week's announcement that China and Hong Kong investors can buy certain stocks listed on each other's bourses.
By midday, the Hang Seng Index was up 0.6 percent at 22,813.92 points. The China Enterprises Index of the top Chinese listings in Hong Kong gained 0.5 percent.
The CSI300 index of the largest Shanghai and Shenzhen A-share listings was up 0.3 percent, while the Shanghai Composite Index was also up 0.3 percent at 2,107.06 points.
Official data showed China's economy grew 7.4 percent during the first three months of 2014, its slowest pace in 18 months, but not significantly lower than the 7.5 percent target top policy makers have set for annual GDP growth.
"The numbers are mostly in line with expectations, particularly with GDP, 7.4 percent is not too bad and is within the range Li Keqiang mentioned in Hainan," said Steven Leung, sales director at UOB Kay Hian in Hong Kong.
Mainland-listed securities firms were up sharply by the lunch break, pushing the CSI300 financial sub-index up 0.8 percent and helping it claw back some of the 2.3 percent the sector lost the previous day.
Founder Securities Co Ltd was up 4.5 percent, Guoyuan Securities Co Ltd rose 3.3 percent, and Industrial Securities Co Ltd gained 2.5 percent.
While continued reaction to last week's news remains the underlying reason for price volatility, the strength of the swings is a reflection of overall uncertainty within the market, analysts said.
"In reality the current market as a whole is quite weak, so securities brokerage stocks are relatively flexible," said Du Changchun, an analyst at Northeast Securities in Shanghai.
"Investors think that if there are new policies to stabilise the economy and re-invigorate the market, these stocks will be the most affected."
It was a mixed day for Hong Kong listed auto shares, with some companies gaining while others continued to head into negative territory after losses on Tuesday, with investors appraising company-by-company growth prospects.
Guangzhou Automobile Group Co Ltd gained 1.9 percent and BYD Co Ltd was up 1.0 percent.
But Great Wall Motor Co Ltd, and Dongfeng Motor Group Co Ltd continued to fall, losing 1.0 percent and 0.5 percent, respectively. (Reporting By Natalie Thomas; Editing by Jacqueline Wong)