Bangkok, Thailand (CNN) -- Two explosions rocked an anti-government protest in the Thai capital of Bangkok on Friday, wounding more than two dozen people, security and health officials said.
The blasts went off as protesters were marching in the streets, said Lt. Gen. Paradon Pattanathabut, the national security chief. The cause wasn't immediately clear.
Twenty-eight people were wounded and taken to hospitals, according to the Erawan Emergency Center, a medical unit monitoring the political unrest in Thailand.
Demonstrators demanding the resignation of Prime Minister Yingluck Shinawatra began their latest round of rallies -- dubbed the "Bangkok shutdown" -- in the capital Monday. But their numbers have shrunk over the course of the week.
Roughly 12,000 protesters were on the streets Friday, Paradon said. That's a fraction of the 170,000 people the government estimated had assembled Monday evening. Demonstrators say their numbers are far higher than official estimates.
Outbreaks of violence have flared around the protests this week. On Tuesday night, authorities reported the nonfatal shooting of two protesters, bus burnings, an assault on police officers and an explosive device being thrown at an opposition leader's house.
Akanat Prompan, a spokesman for the main protest group, said the blasts Friday were the first attack on marches in daylight on a public street.
He said the first explosion took place about 30 meters from Suthep Thaugsuban, the leader of the protest group, and generated a lot of smoke.
But the situation wasn't too chaotic, he said. Marchers continued on to the Lumpini Park area, one of their main rallying points.
The protests this week caused some disruption in central Bangkok, but large areas of the sprawling capital city remain unaffected. The government has deployed about 20,000 security personnel throughout the city.
Rights groups, the United Nations and the United States have called on Thai authorities and anti-government protesters to respect human rights and avoid violence during the mass demonstrations.
Since demonstrations against Yingluck's government began in November, eight people have died and more than 450 have been wounded, according to authorities. The country is still scarred by the severe 2010 civil unrest that left about 90 people dead.
In a bid to resolve the current crisis, Yingluck dissolved the nation's parliament last month and called for new elections to be held on February 2.
But the move has done little to appease protesters. They have called on the Prime Minister to step down from her caretaker position and be replaced by an unelected "people's council," which would see through electoral and political changes.
The opposition Democrat Party has said it will boycott the elections.
Protest leaders have said they want to rid Thailand of the influence of former Prime Minister Thaksin Shinawatra, the older brother of Yingluck.
That's an ambitious goal in a country where every election since 2001 has been won by parties affiliated with Thaksin, a billionaire who built his political success on populist policies that appealed to Thailand's rural heartland.
Thaksin was ousted in a military coup in 2006 and has spent most of the time since then in exile overseas. If he returns, he risks a two-year prison sentence on a corruption conviction, which he says was politically motivated.
The recent protests in Bangkok were prompted by a botched attempt by Yingluck's government to pass an amnesty bill that would have opened the door for her brother's return.
That move added fuel for critics who accuse her of being nothing more than her brother's puppet, an allegation she has repeatedly denied.
Opposition to Thaksin and Yingluck is strongest among the urban elites and middle class, particularly in Bangkok.
Thaksin's traditional support comes from the populous rural areas of north and northeast Thailand.
His supporters, known as "red shirts," support the holding of elections on February 2.