Fox hunting with dogs is now illegal in England and Wales after a ban on the activity came into force overnight.
Hunts must now follow a number of alternatives to hunting with hounds
The law faces a stiff test this weekend, with the Countryside Alliance saying many hunts will be out in force.
Chief police officers spokesman Nigel Yeo said he expected most people would obey the law - by drag hunting or chasing foxes then shooting them.
He said police would challenge the "one or two isolated hunts" which are threatening to break the law.
But Simon Hart of the Countryside Alliance has questioned how police will ensure there are no violations.
"The definitions of legal and illegal hunting are so blurred that the police are being asked to make impossible judgements.
"You can hunt a rat, but not a mouse, a rabbit but not a hare, an artificial scent, but not a real one," he said.
The Attorney General, Lord Goldsmith, has so far issued no instructions to police on how they should deal with hunters who violate the law.
He said he will consult the Director of Public Prosecutions and the police "in the near future" to decide what measures to take with regards to hunting prosecutions.
He has rejected a "blanket policy" of not enforcing the ban until the House of Lords has considered its legality.
Anti-hunting activists are celebrating the introduction of the ban.
"We are delighted that 80 years of peaceful, legal campaigning has paid off and that hunters will not be permitted to inflict suffering on wild mammals for their entertainment," a spokesman for the League Against Cruel Sports told BBC News.
"If they are serious when they say hunting is about the chase and not the kill, we will wish them well in their new, cruelty-free pastime of drag-hunting."
'Hunting will return'
The Beaufort Hunt had one pack out on Thursday and has promised a hunt this weekend.
Under the new law hunters have a number of legal options available to them.
As well as being able to mount a hunt for an artificial scent, it will still be legal for the hunts to "flush out" foxes, as long as they shoot their quarry rather than set the hounds on them.
"We are not going away. We will keep these hounds going, we will keep this community going and in the end we will come back and hunt when hunting is legal again," hunt master, Captain Ian Farquhar, said.
But Tony Banks, Labour MP for West Ham, said the issue would soon disappear, and that "people in a few years time will be wondering what it was all about".
He said had the government not prevaricated since 1997 in introducing the ban, hunting with dogs would have passed into history like other former country pursuits such as otter hunting and badger baiting.
"Let the election decide this because the Conservatives have made clear that if they get elected into government they will restore hunting," he said.