Oil has begun to flow from the biggest development project in the North Sea in a decade.
More than 300 million barrels of heavy crude oil are expected to be recovered from the Mariner field, 95 miles east of Shetland.
Norwegian state-owned operator Equinor said the field was a "cornerstone" in its expansion into UK waters.
But environmental charity Friends of the Earth Scotland insisted the oil should be left in the ground.
The field was first discovered in 1981 but was taken over by Equinor, then Statoil, in 2007.
Low flow rates meant it was not economically profitable to recover the oil until technology advanced 25 years later.
Hedda Felin, senior vice president at Equinor, said: "The startup of the Mariner field marks a significant milestone for us as it's our first UK operatorship and Mariner is our cornerstone. So it contributes to our commitment to be a safe and reliable long-term energy partner for the UK.
"We have a lot of new technologies to be applied on the field. We have been able to get much better data of the subsurface so we have been able to increase the recoverable volumes.
"Carbon and climate are at the core of everything we do and we want to push the energy transition in the right way.
"We believe there will be a need for both oil and gas and renewables. There are different opinions on how quick it will go but we will be part of that path."
Production drilling started in December 2016 and more than £6bn has been spent on the project over the past seven years.
This has created 800 construction jobs and a further 700 permanent posts.
At its peak, about 70,000 barrels of oil a day is expected to be produced.
Mike Tholen, from industry body Oil and Gas UK, said: "First oil from Mariner represents a significant achievement for Equinor and its pioneering development of the field, helping the company to become a major player in the UK sector of the North Sea with a long-lasting, positive impact on the region.
"As the largest offshore development in the UK for a decade, Mariner has utilised pioneering technology to bring on stream a field first discovered nearly 40 years ago; in doing so it has provided thousands of highly-skilled jobs across the country during its development and will support many more in the years to come."
Mariner is one of the largest development projects in the North Sea oil and gas sector in a decade.
Production has started just a few months after the huge Culzean field began supplying up to 5% of the UK's gas demand.
Mary Church, from Friends of the Earth Scotland, said: "The Mariner project is seriously bad news for life on earth. We know we have got to stop burning oil and gas if we want to avoid climate breakdown, so every new field like this takes us in the wrong direction.
"Its time for the Scottish and UK governments to stop backing oil and gas expansion and instead redirect support and subsidies towards creating decent jobs in the renewable energy economy."