BRUSSELS — Amid a growing outcry over American snooping on foreigners that threatens to cloud European-U.S. trade talks and President Barack Obama’s visit to Berlin, the European Union’s top justice official has demanded in unusually sharp terms that the United States reveal what its intelligence is doing with personal information of Europeans gathered under the Prism surveillance program revealed last week.

Viviane Reding, the Union’s combative commissioner of justice, told Attorney General Eric Holder in a letter sent on Monday evening that individual citizens of European countries had the right to know whether their personal information had been part of intelligence gathering “on a large scale.”

In the letter, seen Tuesday by the International Herald Tribune, she also asked what avenues were available to Europeans to find out whether they had been spied on, and whether they would be treated similarly to U.S. citizens in such cases.

“Given the gravity of the situation and the serious concerns expressed in public opinion on this side of the Atlantic, you will understand that I will expect swift and concrete answers,” Mrs. Reding wrote.