Thailand was holding re-run elections on Sunday in five provinces where voting was disrupted in last month's poll by anti-government protesters trying to unseat Prime Minister Yingluck Shinawatra and officials said all was going smoothly.

Dozens of gunshots and at least two explosions raised tension on the eve of the general election, which was seen as incapable of restoring stability in deeply polarised Thailand whatever the result.

Protesters were gathering in central Lumpini Park in Bangkok on Sunday, where many already sleep in tents alongside boating lakes, after protest leader Suthep Thaugsuban said they would abandon other sites in the city.

Some of the other protest sites were being dismantled but the elevated highway along the south side of Lumpini Park on the edge of Bangkok's financial district was still blocked.

Election re-runs planned for April in other provinces have been suspended pending a court decision on procedures.

Voting was disrupted in 18 percent of constituencies, 69 out of 375, nationwide, the Election Commission said, affecting 18 of 77 provinces.

Commission secretary-general Puchong Nutrawong said voting had gone ahead peacefully on Sunday.

The demonstrators, who have blocked intersections in the capital for weeks, say Yingluck must resign and make way for an appointed "people's council" to overhaul a political system they say has been taken hostage by her billionaire brother and former premier, Thaksin Shinawatra.

The election is almost certain to return Yingluck to power, thanks to her support base in the largely rural north and northeast, a result the opposition will never accept.

The result cannot change the dysfunctional status quo in a country popular among tourists and investors yet blighted by eight years of turmoil, pitting the Bangkok-based middle class and royalist establishment against the mostly poor, rural supporters of the Shinawatra family.

"For those who worried that I may give up, I can reassure that an old man like me does not know how to give up," Suthep said on Saturday, in reference to the scaled-down protest.

"That's because there are millions of people who support me ... This is not a move to retreat, but it is an adjustment in our manoeuvre for fighting."

Protest numbers had dwindled amid attacks on various camps with grenades and guns. Three people were killed when a grenade was thrown into a busy shopping area near one camp last Sunday.

But most of the city has been unaffected. In total, 20 people have been killed in protest-related violence in Bangkok since November 30 and three in the eastern province of Trat.