KARACHI, Pakistan (Reuters) - Six people died overnight in Karachi, police and ambulance men said, raising to 17 dead the toll from hours of apparent tit-for-tat killings by ethnic rival factions. At least 12 people were killed in separate incidents on Monday evening and four were shot dead overnight. Another bullet-riddled body was also found. Angry mobs torched at least three vehicles, threw stones, blocked traffic and forced shops to shut down, and bursts of gunfire were heard frequently. Witnesses said paramilitary rangers with machine-guns mounted on their vehicles were patrolling sensitive areas of eastern and central Karachi where the violence was centred. A senior police official said the killings appeared to be between rival factions of the ethnic parties. The killings started on Monday evening with an attack on the party office of Mohajir National Movement (Haqiqi) in which masked gunmen shot dead three Haqiqi workers, including a prominent member, Imtiaz Ahmed. A spokesman of a mainstream rival faction, Mutahida Qaumi (National) Movement (MQM), denied his party was involved but said three brothers, including an 11-year old, belonging to the MQM were killed inside their home by gunmen. Both groups have engaged in bloody conflict for the last several years seeking to control Karachi, the country's commercial capital. Ambulance workers said two of those killed during the night were Mutahida National Movement workers. The others were not immediately identified. MQM speaks for the Urdu-speaking people who migrated from India at partition in 1947. MQM Haqiqi is a breakway faction of the MQM. Each faction has blamed the other for the killings and each deny the charges. More than 80 people have been killed in Karachi's political, ethnic and sectarian violence so far this year.