* CSI300 +5.8 pct; SSEC +5.2 pct; HS I +2.1 pct
* Most China shares rise 10 pct daily limit for second day
* Worst-case scenario already priced in China H-shares - UBS
By Samuel Shen and Pete Sweeney
SHANGHAI, July 10 (Reuters) - China stocks surged for the second day, reversing an early-week slump in frenetic trading on Friday as markets regained a measure of composure following a barrage of government support measures to stem the rout.
The CSI300 Index of China's biggest listed companies jumped 5.8 percent by midday, after Thursday's 6.4 percent rise, heading for a weekly gain of over 6 percent.
The Shanghai Composite Index was 5.2 percent firmer at the end of the morning session, also poised to end the week up around 6 percent.
The positive sentiment rippled to Hong Kong, where stocks gained over 2 percent.
"Chinese investors move in herds. After panic selling drove the market down to the extreme, prices are now starting to move in the other direction," said Samuel Chien, a partner of Shanghai-based hedge fund manager BoomTrend Investment Management Co.
The relative calm in mainland markets followed a barrage of government measures this week as policy makers scrambled to put a floor under a stock market that had tumbled around 30 percent in just three weeks, wiping trillions of dollars off the market value.
These measures include banning listed companies' big shareholders from selling shares over the next three months, directly buying stocks and mutual funds through China's state margin lender with liquidity support from the central bank, and limiting shorting activities in stocks and stock index futures.
Only one stock, the loss-making trading firm Jiliin Chengcheng Group fell on Friday, while shares of 1,458 listed companies rose, most of them hitting the upward limit of 10 percent.
Around 50 stocks resumed trading on Friday, although trading in about half of China's listed companies remained suspended.
Banking heavyweights underperformed the broader market, with the CSI300 bank index up only 3.6 percent.
Hong Kong stocks tracked gains on the mainland, with financial and energy shares rising sharply.
The Hang Seng index added 2.1 percent, to 24,910.28 points, and the Hong Kong China Enterprises Index gained 4.3 percent, to 11,939.99.
"We are seeing the near term effect of the (central) government's supportive measures," said Conita Hung, a director at Hong Kong-based Amicus Asset Management.
UBS strategist Lu Wenjie said in his latest strategy report that liquidity in the mainland stock market should improve as the state buys more stocks, and the systematic risk is minimal.
In the H-share market, or Chinese companies listed in Hong Kong, "market valuation has priced in the worst scenario." (Additional reporting by Donny Kwok; Editing by Shri Navaratnam)