TUNIS - Tunisia's largest union cancelled on Wednesday a general strike set for the next day, a move which could calm Islamist-leftist tensions after violent clashes in the capital.The UGTT union announced the strike last week after several hundred Islamists armed with knives and sticks charged a union gathering and smashed office windows on December 4.Police had to intervene to separate the two groups."The strike was cancelled because of a lack of security," UGTT Secretary-General Hussein Abassi said, without giving more details.The moderate Islamist Ennahda party won elections last year, following the 2010 ousting of veteran leader Zine al-Abidine Ben Ali, but has been struggling to restore security and stability.A policeman was killed in clashes with suspected Islamist fighters near the border with Algeria on Monday, stoking fears of a spread of militancy in the north African nation.