Small-time investors in Hong Kong were left reeling on Monday as US President Donald Trump's punishing tariffs and Beijing's retaliation saw the city's stock market suffer its worst day in almost three decades.

The benchmark Hang Seng Index fell by 13.2 percent – its biggest drop since 1997 during the Asian financial crisis.

At a securities brokerage in Central, where more than a dozen elderly investors stared at numbers flashing red on computer screens, the mood was grim.

A woman in her nineties, Tam, said she "hated" Trump.

"He cost me HK$200,000," she said.

"He's nonsensical, he says one thing and changes his mind a few minutes later... How can someone in such a lofty position act like that?"

"[Trump] won't let it go, he's making a mess," said another retiree Lee.

"Everyone around me is losing money."

Hong Kong resumed trading on Monday after a three-day break, which worsened the drawdown, according to Stanley Chik, head of research at Bright Smart Securities.

Hong Kong's stock market had outperformed the United States since Trump took office, but Monday's rout wiped out HSI gains from the first quarter.

Investors have taken a wait-and-see approach for weeks as Trump finalised his trade policies, Chik said, adding that the mood was not yet one of "despair".

Hong Kong tops the world in retail investor participation, with one 2023 survey showing that 48 percent of the respondents held or traded stocks in the preceding year.

A 35-year-old man, Tsang, said his long-term investments lost around US$12,900 on Monday, but he would not consider selling yet.

"I didn't expect it to get so bad," said Tsang, a bank employee.

China A-shares may be more resilient, he added.

"In this sort of fight (between China and the United States), it's hard to say who will suffer more."

Lawyer Ray Chan, 30, was among those left unscathed on Monday, as he sold all his Hong Kong and US shareholdings two weeks ago, netting gains in the seven figures.

"We're clearly entering a bear market but I'm prepared," Chan said.

"When [Trump] said there would be tariffs on April 2, I could guess where things were headed."

It will take "at least a year" before he returns to the market, Chan said. (AFP)