“The bomb went off about 30 metres from Suthep and then his bodyguards escorted him back to a rally stage,” spokesman Akanat Promphan told reporters.

Television footage showed several people lying on the ground as ambulances rushed away the wounded. Protesters were seen searching nearby buildings for the attackers.

Police were investigating what type of explosive device caused the blast.

An official from the city’s Erawan emergency centre said at least 28 people were hurt in the explosion, without giving details of the injuries.

Video: Bomb hits Thai protest march, 28 wounded

Eight people have been killed and hundreds injured in street violence since the protests began.

There have been a series of drive-by shootings at rally sites and grenade attacks on the houses of opposition politicians that both the demonstrators and the government have blamed on each other.

“Yingluck must take responsibility,” one of the protest leaders, Satit Wonghnongtaey, said on stage soon after the blast.

“This government, Yingluck and Red Shirt thugs are creating violence,” he said, referring to a rival pro-government protest movement whose rallies in 2010 were suppressed in a bloody military crackdown.

The government denied the claim, saying the protesters were trying to incite violence.

Meanwhile, a senior Thai minister said on Friday “it’s about time” to take back control of the capital, Bangkok, a sign the government may be losing patience with the blockade.

Foreign Minister Surapong Tovichakchaikul was speaking as a delegation of officials, escorted by police and the military, set out for a government office that issues passports to persuade protesters there to leave and allow work to resume.

“If successful, this can be an example for other ministries to follow,” Surapong told a news conference.

Asked if the government was now moving to end a blockade of ministries and several key intersections of the city, he said: “Soon. It’s about time. We have to start to do something.”

Hundreds of people on motorbikes and in other vehicles drove up to a government administrative area occupied by protesters and a confrontation ensued between two groups, National Security Council Secretary-General Paradorn Pattanatabut told reporters.

“They said they were angry at the anti-government crowd who blocked traffic there and stopped them from getting access to government services, especially the passport office,” he said.

The political unrest flared in November and escalated on Monday when demonstrators led by former opposition politician Suthep Thaugsuban brought parts of the capital to a standstill, though the numbers protesting appeared to be dwindling by the middle of the week.

The turmoil is the latest episode in an eight-year conflict that pits Bangkok’s middle class and royalist establishment against the mostly poorer, rural supporters of Yingluck and her brother, former premier Thaksin Shinawatra.

Speakers at protest sites across central Bangkok have suggested that Yingluck is worn out and eager to quit, but at a news conference on Friday the prime minister maintained that she still enjoyed overwhelming popular support.

Thaksin’s rural and working-class support has ensured he or his allies have won every election since 2001 and Yingluck’s Puea Thai Party seems certain to win the February 2 election.

The anti-government protesters have rejected the election.

They want to suspend what they say is a fragile democracy destabilised by Thaksin, whom they accuse of nepotism and corruption. Their goal is to eradicate the political influence of his family by altering electoral arrangements, though in ways they have not spelt out, along with other political reforms.

Many ministries and state agencies closed this week to avoid violence, with staff working from home or back-up facilities.

The protesters have set out to paralyse ministries, marching each day from camps they have set up at seven big intersections. On Thursday, they targeted revenue offices.

Foreign Minister Surapong said this week that the closure of the consular department in the main government complex meant that, in the first four days of this week, 16,000 new passports had been delayed.

The security forces have largely kept out of sight since the blockades began this week, with the government keen to avoid any confrontation.

The unrest is hurting the economy. Finance Minister Kittirat Na Ranong said this week it might only grow 3 per cent this year rather than the forecast 4.5 per cent because of disruption to manufacturing, exports, consumption and tourism.