TEHRAN — Iranian security forces arrested a prominent reformist politician on Monday, hours after he held a news conference thanking President Hassan Rouhani for a more open political atmosphere, several Iranian news agencies reported.

The arrest of the politician, Ali Shakorirad, put a damper on the hopes of Iran’s reformists. Just last week they started the Islamic Iranian National Union Party, filling a void after the two original reformist parties were banned by Iran’s judiciary after antigovernment protests in 2009.

The arrest illustrates the uphill political battle Mr. Rouhani faces in executing his agenda of more political and personal freedoms.

Mr. Shakorirad has long played a central role in a movement calling for democratic political reforms in the Islamic republic. While many of his fellow critics received long jail sentences in 2009, Mr. Shakorirad, a physician, was released from a brief detention after pressure from reformist Shiite Muslim clerics.