BEIRUT, Lebanon — Syrian insurgents fighting to topple President Bashar al-Assad struck at high-profile targets in the capital region on Thursday for the third time this week, demonstrating their increasing effectiveness and reach.

The latest attack was a double bombing. One bomb was detonated in the parking garage of the Palace of Justice in downtown Damascus, according to Syrian state television, and the other at a city police station, according to local residents. The day before, an attack destroyed another pro-government television station, and late Monday the opposition Free Syrian Army struck the barracks of the elite Republican Guard, next to the palace of President Assad.

These assaults followed a wave of high-level military defections from Mr. Assad’s forces, and a surprise visit by the former head of the opposition Syrian National Council, Burhan Ghalioun, who crossed into Syria and toured what he called liberated territory in Idlib, a city near the Turkish border.

While none of these developments were militarily decisive, they have helped build a public perception that the opposition, while still clearly an underdog fighting a large military machine, was finally making some headway.