A string of small but persistent and violent protests in Yemen is putting fresh pressure on President Ali Abdullah Saleh, who only last week appeared to have outmaneuvered political opponents by dialing back popular unrest.

On Tuesday, security forces dispersed hundreds of protesters for a fifth consecutive day, after pro-government and antigovernment demonstrators clashed in the capital San'a.

The series of protests, which began Friday evening as a small demonstration in celebration of the resignation of Egypt's Hosni Mubarak, has swelled into daily, boisterous and violent clashes.

Mr. Saleh, in power for more than 30 years, is a key U.S. ally in its fight against al Qaeda, which has orchestrated a series of recent, sophisticated attacks from Yemeni soil. Washington has significantly boosted military aid to Yemen to help combat the local affiliate of the international terror group.