phnom penh cambodia main opposition party monday put banners around country call release detain leader kem sokha challenge government prime minister hun sen accuse treason kem sokha leader cambodia national rescue party cnrp arrest september charge plot state western countries human right group condemn arrest say raise severe doubt whether next year general election fair ballot hun sen could face great electoral test three decades power party banners picture kem sokha wave vehicle last election campaign 2013 call leader immediate unconditional release hun sen say opposition must drop kem sokha choose new leader face total ban put banners put banners branch provinces say pol ham deputy leader party son chhay another senior member party say crisis worsen talk need resolve evidence present kem sokha far video record 2013 discuss strategy win power help u.s. experts hun sen say rival get help unite state u.s. embassy reject suggestion interference politics government spokesman phay siphan say opposition party banners amount pressure court attempt interfere work unacceptable say court schedule hold hear determine legality kem sokha detention tuesday cambodia 's opposition put banners call leader 's release