People in Kuwait are voting for a new parliament for the second time this year, amid calls for a boycott.
The opposition says new voting rules decreed by the Gulf state's emir favour pro-government candidates.
Kuwait has had months of confrontations between the opposition and government, but the latest protests have been peaceful.
Correspondents say the demonstrators represent a cross section of Kuwaiti society - Islamists, tribal elders, young men and women, and parents who even brought their children.
Women in black Abaya dresses and full face cover marched next to young women in jeans and t-shirts.
Kuwaitis are casting their votes for the second time this year as they head to the polls to choose a new parliament amid growing unrest.
On the eve of the election, tens of thousands of protesters in Kuwait City called for a boycott over changes made to the voting rules last month.
Opposition MPs say the amendment manipulates the ballot in favour of pro-government candidates.
Kuwait has had months of confrontations between the opposition and government.
Polling stations in the affluent Rumaithiya district of the capital appeared busy on Saturday morning, despite the calls for a boycott, the BBC's Shaimaa Khalil reports from Kuwait City.
But the AFP news agency reported a far thinner turnout at a polling station in Salwa, 15km south of Kuwait City.
The main opposition grievance is a 19 October decree ordered by the emir, Sheikh Sabah al-Sabah, whose family dominates Kuwait's government.
The crisis was sparked in June, when the Constitutional Court annulled parliamentary elections held in February, in which the Islamist-led opposition made significant gains. The court also reinstated the previous assembly, allied to the ruling family.
After months of protests, Kuwait's emir ordered the dissolution of that parliament and announced new elections.
That decree cut the number of candidates a voter can elect from four to one, saying it would ensure a fairer representation of people in the parliament.
But critics of the amendment say it gives the government greater influence over the outcome of the ballot.
Opposition MPs say the changes breach the Gulf state's constitution. As a result they decided not to participate in the election.
Friday's protesters were angry at what they say is a unilateral decision by the emir to skew the election, which will not create a parliament representing the people, the BBC reports.
This level of political polarisation is unusual in Kuwait, which has traditionally had a more unified political scene.
Carrying banners reading "absolute power corrupts", demonstrators marched through Kuwait City chanting, "we are boycotting" and "the people want to bring down the decree".
The rally was led by former Islamist MPs, by liberals and by young people, our correspondent says, adding that the mood was jubilant but defiant.
Unlike recent unauthorised protests, which ended in clashes between protesters and police, authorities had issued a permit for Friday's peaceful march.
Kuwait's parliament has the most powers of any elected body in the Gulf and opposition MPs openly criticise the ruling Sabah family.
However, the Sabahs retain full control over key government and executive posts.
The emir has dissolved parliament four times since 2006.