"We have concerns when people who are trying to express their views, and do it in an open and democratic way, are not able to do so," Powell said on Friday after talks with Saudi Arabia's de facto ruler Crown Prince Abd Allah.
"We had a good, candid, open debate about this issue," Powell said.
The US, in a rare public criticism of Saudi Arabia on Thursday, had condemned the detention of at least 10 reformist figures, including former university professors, as a step backward for political progress.
Saudi Arabia insisted the arrests were an internal matter.
"What happened was something which concerned our country," Foreign Minister Prince Saud al-Faisal bin Abd al-Aziz Al Saud said.
"These people sowed dissension when the whole country was looking for unity and a clear vision, especially at a time when it is facing a terrorist threat," he said.
Several of the detainees have since been released.
Their supporters said they were arrested because they had called for the absolute monarchy to move towards a more constitutional model and were planning to criticise a state-approved human rights group set up earlier this month.
A senior US official travelling with Powell said the Saudi foreign minister had informed them that just three or four were still in detention and that "some might be charged".
"What happens to those who remain we will see in the days ahead, but we have expressed our opinion on this one," said Powell.