Two explosions have hit an anti-government demonstration site in Bangkok, wounding at least 28 people, as Thailand's increasingly volatile political crisis drags on.

Police said the blasts on Sunday near Victory Monument, in the north of the capital, were caused by fragmentation grenades – the same kind that killed one man and wounded dozens in a similar explosion targeting protest marchers on Friday.

The demonstrators, who control several small patches of Bangkok, were vying to overthrow the prime minister, Yingluck Shinawatra, and her government and derail elections on 2 February, which she called in a bid to quell the crisis.

Witnesses said the explosions occurred about two minutes apart. The first blast went off less than 200 metres from a stage set up by protesters, leaving a small crater beside a vendor's stall. The second went off near a row of stall selling anti-government T-shirts, leaving bloody clothes and ripped white-and-blue tarpaulin scattered across the ground.

Protester Theerayuth Uthakapintanont said two vendors, who were selling merchandise to demonstrators, were hit by the second blast. The Erawan medical centre, which tracks casualties, said 28 people were wounded.

Such incidents have occurred nearly every day during the past week in Bangkok, including shootings at protest venues and bombings at the homes of leading supporters of the protests.

It is unclear who is behind the unrest. But prolonged violence, even on a small scale, increases the chance that the Thai military will stage a coup. Such a scenario would benefit protesters, who have called on the army to take sides and do not have the numbers to bring down the government on their own.

Thailand's army has staged about a dozen coups since the end of absolute monarchy in 1932. The last coup, in 2006, toppled the then prime minister, Thaksin Shinawatra – Yingluck's brother – and prompted a societal schism that in broad terms pits the majority rural poor who back the Shinawatras against an urban-based elite supported by the army and royalists who see Yingluck's family as a corrupt threat to the country's traditional power structures.

Yingluck's opponents – a minority that can no longer win at the polls – argue the Shinawatras are using their electoral majority to impose their will and subvert democracy. The crisis boiled over again late last year after the ruling party attempted to push through an amnesty bill that would have allowed Thaksin to return from self-imposed exile. He has lived overseas since 2008 to avoid a prison sentence for a corruption conviction.

Anxious about triggering military intervention, Yingluck has ordered police to go out of their way to avoid confrontations with protesters. The strategy is aimed at averting violence but it also has undermined rule of law and the government's authority, with police staying away from the scattered pockets of Bangkok controlled by demonstrators.

The protest movement has taken the law into its own hands. A protest leader, Issara Somchai, said demonstrators on Saturday detained two men allegedly found with small homemade explosives and handcuffs. Somchai said on Sunday that the pair, suspected of planning violence, were still in the hands of protesters and were being "investigated".

"We are taking care of them. They are safe with us," said Somchai, adding that the men were also being protected from demonstrators who could seek revenge.