HONG KONG, Sept 25 (Reuters) - The publisher of Hong Kong's Apple Daily controlled by pro-democracy tycoon and Beijing critic Jimmy Lai on Wednesday condemned an assault on one of its reporters by unknown assailants at a restaurant.

The female reporter has been covering anti-government protests that have plunged the former British colony into its biggest political crisis in decades and posing a direct challenge to the central government in Beijing.

The journalist was attacked at a restaurant on Tuesday night by four men dressed in black and wearing the protesters' trademark yellow helmets and masks, Apple Daily reported.

The reporter was taken to hospital, the newspaper said, but it gave no details on her condition.

Publisher Cheung Kim-hung said the newspaper "has no fear of violence and will continue to defend freedom of the press and the public's right to know".

The Hong Kong Journalists' Association condemned the attack, which police said was under investigation.

The incident follows an assault on democratic lawmaker Roy Kwong, who was taken to hospital on Tuesday after being punched and kicked by three men in the Tin Shui Wai district close to the border with mainland China.

Fellow Democratic Party lawmaker Lam Cheuk-ting said the assailants had suspected links to triads, organised criminal gangs, and the attacks were intended "to send a message to threaten all" pro-democracy lawmakers.

Demonstrators are frustrated at what they see as Beijing's tightening grip over the city, which returned to China in 1997 under a "one country, two systems" formula intended to guarantee freedoms not enjoyed on the mainland.

China has said it is committed to the arrangement and has repeatedly warned foreign countries not to interfere in an "internal affair".

Speaking at the U.N. General Assembly in New York, U.S. President Donald Trump drew a link between resolving the U.S.-China trade dispute and Beijing's treatment of Hong Kong.

Trump said Washington was "carefully monitoring the situation in Hong Kong".

"How China chooses to handle the situation will say a great deal about its role in the world in the future. We are all counting on President Xi as a great leader," he said, referring to his Chinese counterpart Xi Jinping.

(Reporting By Donny Kwok and Anne Marie Roantree; editing by Darren Schuettler)