Bhutan's proposed constitution is aimed at deflecting international attention from the country's refugee crisis, exiled political groups say.
About 100,000 refugees from Bhutan live in camps in Nepal
The National Front For Democracy coalition said the royal government was trying to impress foreign donors, but was planning only "limited democracy".
Bhutan will become a two-party state if the draft constitution, unveiled a week ago, is approved in a referendum.
Over 100,000 refugees, mostly Hindus of Nepalese origin, left Bhutan in 1990.
Their departure followed alleged political repression by the government of the Buddhist kingdom, which has an absolute monarchy.
Bhutan says most of the people in the camps are illegal Nepalese immigrants and it is only prepared to allow back a few thousand of them whom it regards as genuine Bhutanese refugees.
The NFD, a coalition of three exiled Bhutanese political groups, told the BBC there could be no democracy in Bhutan until the refugees returned home.