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."
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.