By Richard Silk 

BEIJING--A closely watched gauge of activity in China's manufacturing sector rose in October, a result which may go some way in easing concerns over the pace of Chinese economic growth.

The preliminary HSBC China Manufacturing Purchasing Managers Index rose to 50.4 in October, compared with a final reading of 50.2 in September, HSBC Holdings PLC said Thursday. A reading above 50 indicates expansion from the previous month, while one below 50 means contraction.

The Hong Kong stock market briefly rallied on the news, before quickly reversing as investors digested the details of the report.

"While the manufacturing sector likely stabilized in October, the economy continues to show signs of insufficient effective demand," HSBC's chief economist, Qu Hongbin, said in a statement.

Worries about China's growth have been building since the beginning of the year, amid a drawn-out slump in housing prices and worries about out-of-control lending. The export sector, long a crucial engine of China's economy, has also suffered as the rest of the world struggles with its own economic problems.

"Exports are still better than the domestic front, but it's a seesaw," said Steve Wang, an economist at Reorient Financial Markets in Hong Kong. "IT, semiconductors and technology seem to be doing well."

Economic growth in the third quarter, reported earlier this week, fell to 7.3% year on-year, its lowest level in over five years, from 7.5% in the second quarter. While that figure is good by most standards, it marks China's poorest showing since 2009.

Many economists expect the government to continue propping up the economy, drawing on the playbook of infrastructure investment and cheap money it has used in the past.

"The government has already started doing that, with newly started railway projects and airports," said CIMB economist Fan Zhang. The whole package may end up matching the size of the second-quarter's stimulus measures, which succeeded in achieving a short-lived rebound in the economy, he said.

Recently there have been some signs of a turnaround. Industrial production was up 8% year-over-year in September, compared with an unusually weak 6.9% in August. The decline in property sales also slowed marginally in September.

The PMI is the first major indicator of the economy's performance this month. The subindexes for input and output prices both declined, indicating that weak demand continues to put a damper on inflation, while the employment sub-index improved.

The preliminary PMI figure is based on 85% to 90% of total responses to HSBC's PMI survey each month, and is issued about a week before HSBC's final PMI reading.

--Liyan Qi and Mark Magnier contributed to this article.

Write to Richard Silk at richard.silk@wsj.com