Six members of Cymdeithas yr Iaith Gymraeg blocked the entrance to the building at Llandudno Junction at about 06:30 GMT.
North Wales Police were called to the protest, which ended at midday.
The Welsh government said it was taking action to promote the language.
The protestors said the action was part of a general Cymdeithas campaign to put pressure on the Labour administration in Wales to act urgently for the Welsh language.
They want the Welsh government to adopt six basic policy changes, including making Welsh-medium education available for all and what they call "fair funding" for the language.
Robin Farrar, chairman of the society, said: "We're here to speak out for the present generation, and those to come, who should be able to live in Welsh.
"There's a real crisis facing the language, but the Welsh government is not acting seriously.
"We're certain that our unique national language can thrive over the years to come with positive campaigning and political will."
A Welsh government spokesperson said: "We are very disappointed that Cymdeithas yr Iaith has decided to take this course of action, especially as we've held regular, and constructive, dialogue with the group.
"Over the past year we have taken positive action to promote the Welsh language, including significant measures such as publishing proposed standards to improve Welsh language services to citizens."

The 17-time Grand Slam champion beat France's Gasquet 6-4 6-2 6-2 to give the Swiss an unassailable 3-1 lead.
"This one is for the boys," said Federer, who was in tears at the end of the match.
"This is not for me, I have won enough. I am just happy we can give everyone in our country a historic moment."
Federer, 33, also paid tribute to compatriot Stan Wawrinka, who helped set up the victory in Lille by beating Jo-Wilfried Tsonga before partnering Federer to win the doubles.
Federer and Wawrinka were involved in a row at the ATP Tour Finals in London, which the world number two later described as "heat of the moment".
"Stan has put in so much effort over the years. He played an unbelievable weekend and that is what gave me the opportunity today," said Federer.
Federer, who lost his singles rubber to Gael Monfils on Friday, played an inspired match as he crushed Gasquet, who replaced Tsonga due to an arm injury.
In front of a passionate crowd of 27,448 - a world record for a competitive tennis match - Federer dropped only four points on his imperious serve in the opening set, which he sealed with a whipping forehand winner.
World number 26 Gasquet dropped serve in the first and seventh games of the second set, which Federer took without conceding a single break point.
He broke the Frenchman twice in the final set to serve for the match and claimed victory with a deft backhand drop shot - collapsing in tears onto the clay at the Stade Pierre-Mauroyto.
The Swiss win comes a week after Federer pulled out of the Tour Finals title match against Novak Djokovic at the O2 in London to protect a back injury.
Welsh language campaigners have chained themselves to the gates of Welsh government offices in a protest at a "lack of response" to the decline in the number of Welsh speakers.
[NEXT_CONCEPT]
Roger Federer beat Richard Gasquet to seal Switzerland's first Davis Cup triumph, then dedicated the victory to his team-mates.