NEW : 15 Egyptian officers forced to leave square , witness says . Numbers of demonstrators have swelled because Groups has joined . Groups are seeking prosecution of former president Mubarak . Within moments , members of the crowd began pushing the officers and knocking their hats off their heads . Hosni Mubarak 's chief of staff has been arrested .
NEW : 15 Egyptian officers forced to leave square , witness says . Numbers of demonstrators have swelled because Groups has joined . Groups are seeking prosecution of former president Mubarak . Hosni Mubarak 's chief of staff has been arrested .