Activists in Yemen say attacks on demonstrators in the capital, Sana'a, have wounded at least four people.

The activists say the protesters were shot in a Sunday rally as they marched past government buildings. Reports vary as to whether the assailants were security forces or loyalists to President Ali Abdullah Saleh.

Protests in the capital are frequent against the longtime president. Demonstrators are calling for his resignation.

The violence comes a day after Yemeni officials said a suicide bomber rammed his explosives-laden car into an army checkpoint in the country's volatile south, killing three soldiers and wounding seven others.

The officials said the bomber drove up to the checkpoint on the road between the port city of Aden and Zinjibar and blew himself up.

Militants have a strong presence in the region, which has seen frequent battles between Yemeni security forces and al-Qaida insurgents.

Unrest caused by militants has increased amid the political turmoil. The embattled president is recovering in neighboring Saudi Arabia from wounds sustained in an assassination attempt in June.