Kuwait's rulers have banned public gatherings of more than 20 people after clashes between riot police and demonstrators over electoral law changes that opposition figures labelled a "constitutional coup".

The protests on Sunday were among the largest ever seen in the Gulf state. The gathering of more than 20,000 people was dispersed with teargas, a rarity in Kuwait, which sees few displays of open dissent towards its dynastic rulers.

Opposition figures and their supporters are angry over moves by the government, which is dominated by the western-backed al-Sabah family, to limit the number of candidates permitted to contest a general election called for 1 December. In past elections, up to four candidates could stand in voting districts, but an official decree has limited that number to one.

The move is thought to be likely to disadvantage opposition groups, who have been energised politically by a series of government crises, most noticeably a corruption scandal earlier this year, and by uprisings in other Arab states.

An affluent and highly organised society, Kuwait, like other Gulf states, has avoided the ramifications of the so-called Arab spring. Its citizens have some of the highest incomes in the world, and have enjoyed several decades of stability.

However, some opposition groups are accusing the government of stripping back political freedoms at a time when communities elsewhere in the region are demanding more such liberties.

Government officials recently said they were wary of "seditious" threats from some members of the opposition, a claim thought to be a reference to Salafist Islamist organisations, which that form a key section of the opposition.

The al-Sabah family dominates executive power in Kuwait, and its authority has remained unchallenged throughout its reign. But on 15 October, a leading opposition politician, Muallem al-Barrak, in effect warned the emir against changing electoral laws. "We will not allow you, your highness, to take Kuwait into the abyss of autocracy," he said in unprecedented public comments. "We no longer fear your prisons and your baton sticks."

Kuwait's parliament has barely functioned this year as ever more assertive and confident opposition groups have clashed with government MPs on numerous issues.

Kuwaitis have throughout remained staunchly protective of their political and consitutional freedoms, and have vowed to continue to challenge the changed electoral laws before the poll.

Some opposition figures say they are willing to force a showdown with the Sabah family by demanding a fully democratically elected parliament.