President Robert Mugabe has urged Zimbabwe's security forces to remain on high alert to thwart attempts to topple his government by the opposition and his Western foes, official media reported on Friday.Mugabe, who in recent months has stepped up warnings against public protests amid an escalating economic crisis, told a ceremony for graduating police officers that threatened strikes and job stayaways were part of a plot by the opposition Movement for Democratic Change (MDC) to sow political turmoil. "Our security forces have heightened their vigilance in order to thwart the subversive manoeuvres of those who engage in crimes of political violence," Mugabe was quoted by the official Herald newspaper as saying at Thursday's event. "I wish to call upon people of Zimbabwe to unite against the shameless British arm-twisting tactics being orchestrated through the MDC and the so-called civil groups," Mugabe said.