Energy firm Cuadrilla said it had started hydraulic fracturing at Preston New Road in Little Plumpton, on Monday, and it will continue for three months.
-> Yorkshire campaigners fury as fracking given green light in LancashireIt is the first time fracking has taken place in the UK since 2011, when the nascent industry was halted after it caused two small earthquakes in Lancashire.
A spokesman for the company said: "Cuadrilla is pleased to confirm that it has started hydraulic fracturing operations at our Preston New Road shale gas exploration site.
"Hydraulic fracturing of both horizontal exploration wells is expected to last three months after which the flow rate of the gas will be tested."
Earlier police closed off the site as about 50 protesters gathered ahead of the start of the process and a team had to cut a man and woman out of a set of tyres which they had apparently cemented their arms into.
One activist sat on top of a van outside the site with a banner which read "Stop the Start" while another lay down in front of railings.
Protester Ginette Evans, 60, from Fleetwood, said: "We'll be monitoring the site 24 hours a day. It is definitely not over, it has just got serious.
"The fight's just really started."
She said the there was a mood of disappointment among protesters.
She added: "I'm gutted really. It's such a dangerous industry and it shouldn't be here."
Justin Vanparys, 47, from Blackburn, said: "Fossil fuels should stay in the ground. We have already got global warming."
He said activists had not been listened to but added: "That just makes you come back even stronger."
The work had been due to start on Saturday but was delayed by the effects of Storm Callum.
It followed an environmental campaigner failing in a bid to halt the work at the High Court on Friday.
At the hearing in London, Mr Justice Supperstone dismissed Bob Dennett's application for an injunction preventing the company from fracking the UK's first horizontal shale gas well pending his proposed legal challenge.
Mr Dennett claimed Lancashire County Council's emergency response planning and procedures at the site were inadequate, but the judge ruled there was not a "serious issue" to be tried which would justify an interim order.
Fracking for shale gas is taking place for the first time in seven years as the UK Government kicks off the first Green GB Week initiative to highlight the green economy.
It comes after a UN report warned of the urgent need to cut emissions to avoid dangerous climate change, and that carbon from electricity, heating homes, transport, industry and land use must reach net zero by 2050.
Craig Bennett, Friends of the Earth chief executive, said: "The world's leading climate scientists have published findings that couldn't be starker: to have a hope of containing global warming we must take action now.
"Yet, ironically, on the same day the government boldly asks how it can reduce carbon emissions to zero, fracking begins.
"It is morally bankrupt to be heralding the start of a whole new fossil fuel industry when climate catastrophe awaits. You can deal with climate change or you can have fracking - you can't do both."