Pakistan gives last warning to undocumented immigrants to leave, in Chaman PESHAWAR, Pakistan, Nov 2 (Reuters) - Thousands of people swamped Pakistan's main northwestern border crossing seeking to cross into Afghanistan on Thursday, a day after the government's deadline expired for undocumented foreigners to leave or face expulsion. Pakistani authorities began rounding up undocumented foreigners, most of them Afghans, hours before Wednesday's deadline. More than a million Afghans could have to leave or face arrest and forcible expulsion as a result of the ultimatum delivered by the Pakistan government a month ago.