Their efforts came as the government said around 1,000 tonnes of the solidified oil had spilled from a cargo ship after a collision with another vessel near the Pearl River estuary in southern China Thursday.
It was the first time authorities had confirmed how much had been spilled.
Around 200 tonnes is likely to reach Hong Kong's shores, deputy environment chief Tse Chin-wan told reporters.
Thirteen of the city's most popular swimming spots are closed after white clumps of the oil started appearing Sunday.
Official cleaning teams have been sent to clear it from beaches and surrounding waters.
But criticism of the government's response is mounting.
On Wednesday afternoon a team of local volunteers from Lamma Island in the south of Hong Kong braved sweltering heat and humidity to comb one of the worst-affected beaches, filling black bin bags with the lumps of oil.
A sour stench hung over the area and small numbers of dead fish were washing in.
The surrounding pathways were slippery with oil that had melted as temperatures hit 33 degrees Celsius (91 degrees Fahrenheit).
"The government should put in more effort to clean up. I'm here because no one else is doing it," Tony Mok, 31, told AFP.
"Every morning it looks like it has snowed in Hong Kong, and every afternoon it's all melted back down under the sand," said Lamma resident Robert Lockyer, who was leading the two dozen volunteers working to remove the oily white clumps.
Aleli Pena, 38, described the clean-up effort as "hard, hot and disgusting".
"We're all volunteers -- where are you government?" she asked.
Stokes, Asia Director for Sea Shepherd Conservation Society, has written to the marine department asking them to install the booms to guard against more oil washing in.
Tse had said earlier that he would not rule out the oil spreading to other areas.
"You will see that many instant noodles have palm oil in them," Tse said Wednesday, although he acknowledged a large amount would affect the environment.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)