The violence has stirred fears of further instability like what plagued the country during related political conflicts in 2006, 2008 and 2010.
In 2008, anti-Thaksin demonstrators occupied Bangkok's two airports for a week after taking over the prime minister's office for three months.
Any escalation is likely to scare away tourists who come to Thailand by the millions and contribute a huge chunk to the economy.
But it may help the government by undermining the claims of its opponents to be carrying out a nonviolent campaign of civil disobedience.
The nighttime clashes involved opponents of the government, led by university students, who tried to block government supporters from entering the rally, which drew more than 50,000 people.
At least some of Sunday's gunshots appeared to have been fired into the nearby university, according to Wutthisak Larpcharoensap, rector of Ramkhamhaeng University.
“Right now there are sporadic shootings into the campus,” Wutthisak said Sunday morning. “Now there are about 2,000 students inside the campus and I'm very worried about the safety of my students.”
Police called for calm in a televised statement, saying they were helping to escort both sides out of the area safely.
Organizers of the pro-government “Red Shirt” rally at the stadium called off the event for safety reasons and sent people home Sunday, after many spent the night camped inside.
Bangkok Emergency Medical Services reported on its website that at least 2 people were killed and 45 wounded.
On Saturday, government opponents had gathered outside the stadium and jeered Red Shirt government supporters.
Two men wearing red shirts were grabbed, one from the back of a motorbike, and beaten.
Two buses were attacked, their windows smashed as passengers cowered inside.
One protester used an iron rod with a Thai flag wrapped around it to smash the driver's side window of one bus.
The buses and one taxi appeared to have been targeted because they carried people wearing red shirts.
The violence capped a week of dramatic protests against Yingluck's government that included seizing the Finance Ministry, turning off power at police headquarters and camping at a sprawling government office complex.
An ill-advised bid by Yingluck's ruling Pheu Thai party to push an amnesty law through Parliament that would have allowed Thaksin's return from exile sparked the latest wave of protests.
Because Yingluck's party has overwhelming electoral support from the country's rural majority, which benefited from Thaksin's populist programs, the protesters want to change the country's political system to a less democratic one where the educated and well-connected would have a greater say than directly elected lawmakers.
Thaksin lives in Dubai to avoid a two-year jail term for a corruption conviction he says was politically motivated.
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