PUERTO ORDAZ, Venezuela — Venezuela’s intelligence police on Thursday detained the chief of staff of the opposition leader, Juan Guaidó, the most significant attack on the opposition camp since Mr. Guaidó declared himself the country’s interim president two months ago.

The United States, a fierce ally of Mr. Guaidó, condemned the arrest and vowed a forceful response. And Mr. Guaidó said the detention would only redouble his efforts to force President Nicolás Maduro to cede power.

“We’re not afraid,” Mr. Guaidó told reporters, surrounded by other opposition leaders. “The only possible escalation here that I can be certain of is to increase street pressure” to oust Mr. Maduro.

Since late January, the Venezuelan opposition has been locked in a standoff with Mr. Maduro that has left the destitute country in the uneasy position of having rival claimants to the country’s leadership. Roughly 50 nations have recognized Mr. Guaidó as Venezuela’s interim leader, even as Mr. Maduro has appeared intent on trying to ride out the crisis and keep his hold on power.