Nearly 10 million viewers flocked to the final-season premiere of PBS drama “Downton Abbey” on Sunday, nearly on par with prior season-openers for the pubcaster’s most popular program of all time.

The British period drama, whose final-season action picks up in 1925, averaging 9.9 million viewers for its 9 p.m. premiere on Sunday, according to preliminary Nielsen estimates compiled by PBS and WGBH Boston, the largest producer of PBS content. Last year’s season 5 premiere drew 10.1 million, and the 2014 debut of season 4 drew 10.2 million.

According to Nielsen, a Twitter audience of 1.4 million people saw more than 47,000 tweets during the premiere evening, generating a total of 6.1 million impressions and making it the No. 1 primetime series episode of the night.

PBS plans to use the final season of “Downton Abbey” to help launch “Mercy Street,” the network’s first U.S.-mounted drama in more than 10 years. “Mercy” will bow Jan. 17, immediately following the third episode of “Downton.”