Two men and two women, aged between 25 and 73, were arrested in Jordan, Kwun Tong and Diamond Hill.
The suspected gang comprised of a retiree, a housewife, a personal trainer and a computer programmer.
Police said they also believe the suspects are linked to “tests on explosives” in Tuen Mun in December, for which three men had already been arrested.
The force said the latest four to be detained were suspected of various offences, including conspiracy to manufacture explosives, conspiracy to wound with intent, and possessing offensive weapons.
Detective chief inspector Lo Chung-wong said police believe the suspects planned to use the weapons at a protest rally over the Lunar New Year period.
“We are quite certain that all these suspects we arrested are involved in the same syndicate and they all have the same intent, which is to cause injury and chaos at public order events,” Lo said.
He added that he did not know when exactly the “attack” was to take place, but police “know” it was being planned.
The force has announced several “weapons finds” in recent weeks, often making the discoveries shortly before large-scale protests are due to take place.