Saboteurs say hunters are exploiting loopholes in the law
The Countryside Alliance says it will continue to press for the ban on hunting with dogs to be repealed.
Spokesman Tim Bonner said the law "fails at every possible level" and things were looking "very positive" for those who wanted to see it overturned.
About 50,000 people are expected to take part in 300 hunts across the UK later on the opening day of the season.
The Hunt Saboteurs Association said "the level of violence from hunts and supporters remains undiminished".
Its members have vowed to carry on their fight to block hunts wherever possible.
Under the Hunting Act, introduced in 2005, dogs can still be used to follow a scent or flush a fox out of cover, but cannot be used to kill it.
The act also allows the hunting of other animals such as rabbits and rats, and states that the law has not been broken if a fox is killed unintentionally.
The HSA says some hunts are using "loopholes" like these to carry on as if the law had never been passed.
Mr Bonner said there was a very strong determination among hunt supporters to keep going.
"The politics of hunting are certainly not dead," he said. "In fact, it's looking very positive indeed as far as the possible repeal of the Hunting Act is concerned.
"Certainly, we need to look at legislation about how mammals are managed in the wild in a consistent and holistic way, but this law has nothing going for it whatsoever.
"It fails at every possible level and we think the only answer is to get rid of it completely."
The HSA says some hunts do obey the rules, but others are showing "a blatant disregard for the law of the land".
The police and criminal justice bodies "do not appear to be too interested," it adds.
Lee Moon, from the HSA, said: "Many [saboteurs] are witnessing not only hunting as usual, but also that the level of violence from hunts and supporters remains undiminished."