SHANGHAI/SINGAPORE (Reuters) - Top Chinese brokerages have pledged to help steady domestic share prices in a concerted effort, the Shanghai bourse said, and scores of listed companies unveiled stock buying plans, as the local market reels from an escalating trade war.

The Shanghai Stock Exchange said late on Tuesday it held a meeting with 10 brokerages to stress the importance of stabilising markets in the face of external shocks.

The participants, including Citic Securities, Orient Securities and Industrial Securities, expressed optimism about China's growth prospects, and vowed to steady the market, it said in a statement.

The United States said on Tuesday that 104% duties on imports from China will take effect shortly after midnight, intensifying trade tensions that have already roiled global markets and smacked Chinese shares.

The brokerage gathering represents an acceleration of efforts by Chinese authorities to try and limit the damage from the trade war, after Central Huijin and several other state-backed investors vowed to increase stock holdings to steady markets.

Separately, more than 100 Chinese listed companies have published announcements regarding share purchases or buybacks to bolster confidence in a market that slumped to six-month lows this week.

Construction machinery maker Sanyi Heavy Industry Co said it bought back 5 million shares worth 92.9 million yuan ($12.64 million) through the public market on Tuesday.

XCMG Construction Machinery said it plans to buy back the company's shares worth up to 3.6 billion yuan.

More than 20 listed companies controlled by the central government unveiled buyback plans under the guidance of China's state asset regulator.

They include prominent oil companies PetroChina and Sinopec, as well as power generators such as China Shenhua Energy Co and GD Power Development.

($1 = 7.3475 Chinese yuan renminbi)

(Reporting by Samuel Shen and Tom Westbrook; Editing by Muralikumar Anantharaman)