The Hang Seng Index on Friday breached the 20,000 level for the first time in more than a year.

The city's benchmark index surged 3.5 percent, or 697 points, to reach 20,621 by noon break, the highest in nearly 18 months.

Half-day turnover was a robust HK$244 billion.

The average daily turnover in August was HK$95.5 billion, according to data from the stock exchange.

The director of asset management at Ample Financial Group, Alex Wong, said the stimulus measures announced by the mainland authorities this week have boosted market sentiment.

"It's due to the stimulus announced by China earlier. It completely turned the sentiment. The market is so strong because so many investors have under-owned Hong Kong equities in the past few years. So they have little exposure in Hong Kong, and actually the buying was mainly by foreigners," he said.

The head of research at Kingston Securities, Dickie Wong, said investors were snapping up shares ahead of the week-long National Day holiday.

"At this current level, I won't say the stock market already hit the ceiling. I truly believe that the momentum may still go through before the National Day holiday – the really long holiday of the mainland investors. Also, the Stock Connect will be closed," he explained.

Both Hong Kong and mainland shares climbed after the People's Bank of China announced lowering the reserve requirement ratio for lenders by 0.5 percentage points with effect from Friday.

The cut came after the authorities announced a batch of other stimulus, including support for the property sector and cuts in mortgage rates, earlier in the week.