Tunisia's powerful labour union said on Monday it would hold a national strike over wages and the economy after refusing to take part in a limited dialogue proposed by the president as he rewrites the constitution. With more than a million members, the UGTT is Tunisia's most powerful political force and its call for a strike may present the biggest challenge yet to President Kais Saied after his seizure of broad powers and moves to one-man rule. Saied has focused on his political agenda since last summer when he brushed aside the parliament and discounted most of Tunisia's democratic constitution to say he would rule by decree despite a gathering economic crisis.