Prime Minister Helle Thorning-Schmidt's assessment of Abdulhadi al-Khawaja's health contrasted with Bahrain's statement that he showed no signs of critical medical problems.
Thorning-Schmidt told reporters Tuesday that Denmark is using its international contacts to press for his release.
"What we're hearing now is that al-Khawaja's condition is very critical. And we continue our efforts at all levels," she said.
Al-Khawaja and other opposition leaders were sentenced to life in prison on anti-state crimes last year as Bahrain imposed martial law to quell political unrest.
Denmark's Foreign Ministry said the Danish ambassador to Bahrain visited al-Khawaja at a military hospital in Manama on Tuesday.
"They talked for about 20 minutes," said Ole Egberg Mikkelsen, a Danish Foreign Ministry official, adding the embassy was awaiting approval for another visit Wednesday.