Troops loyal to President Ali Abdullah Saleh clashed with those supporting Gen Ali Mohsen al-Ahmar, who has defected to the opposition.
In the southern city of Aden, soldiers opened fire as protesters threw stones and set up roadblocks to stop troops patrolling the streets.
One person was killed and several others wounded, reports say.
Tens of thousands of people are marching in cities across the country, as protesters keep up weeks of pressure aimed at forcing President Saleh out of power after 32 years in office.
'Worrying development'The clash in Sanaa between rival security forces happened early on Wednesday morning at a checkpoint run by the first army division, which supports the opposition.
The rival sides exchanged fire with machine-guns and rocket-propelled grenades after the pro-opposition soldiers stopped a car carrying pro-government security personnel, who reportedly refused to be searched.
This is a particularly worrying incident as Yemen's capital is guarded by two rival military units, and any confrontation between them could be very difficult to contain, says a BBC correspondent in Sanaa.More than 100 people have been killed since the start of the protests on 11 February, which were inspired by the popular uprisings that toppled long-time rulers in Tunisia and Egypt.
In addition to democratic and economic reforms, the protesters want to see legal action against Mr Saleh and his sons, who occupy key security and political posts.
On Monday, opposition groups rejected outright a proposal by Gulf Arab countries for Mr Saleh to transfer power to his deputy in exchange for immunity from prosecution.
They now say they will hold talks with Gulf ambassadors to see if a timetable can be agreed and other details ironed out.
Even before the mass protests, Mr Saleh was struggling to quell a separatist rebellion in the south and a Shia Muslim insurgency in the north.
Analysts fear the violence could give the Arabian Peninsula branch of al-Qaeda more room to operate.
Send your pictures and videos to yourpics@bbc.co.uk or text them to 61124 (UK) or +44 7725 100 100 (International). If you have a large file you can upload here.
Read the terms and conditions
View the original article here
This post was made using the Auto Blogging Software from WebMagnates.org This line will not appear when posts are made after activating the software to full version.
Protests have been continuing for weeks Hundreds of anti-government protesters are reported to have been hurt in the Yemeni capital Sanaa, a day after similar clashes in the city of Taiz. 
Anti-government protesters believe the concessions offered do not go far enough Yemen's President Ali Abdullah Saleh has ordered his security forces to offer "full protection" to anti- and pro-government demonstrators alike. 
President Ali Abdullah Saleh in power since 1978Population 24.3m; land area 536,869 sq kmThe population has a median age of 17.9, and a literacy rate of 61%Youth unemployment is 15%Gross national income per head is $1,060 (World Bank 2009)Thousands have demonstrated daily in these and other Yemeni cities over the past few weeks, demanding that the president step down and that unemployment and corruption be tackled.