The Associated Press gained rare access to the city, which was the Congo government's last major foothold in South Kivu province after the provincial capital of Bukavu fell to the rebels in February. Its capture allows the rebels to consolidate a broad corridor of influence across the east.
M23 said it took control of Uvira earlier this week, following a rapid offensive launched at the start of the month. Along with the more than 400 people killed, about 200,000 have been displaced, regional officials say.
On Saturday, the situation in Uvira still had not returned to normal.
There was absolute silence and no traffic, apart from military jeeps circulating on the empty streets. The banks were closed and people have not resumed their jobs - only a few dared to go out during the day, and no one ventured outside after sunset, with armed M23 fighters patrolling the city.
The rebels' latest offensive comes despite a US-mediated peace agreement signed last week by the Congolese and Rwandan presidents in Washington.
The US accused Rwanda of violating the agreement by backing a deadly new rebel offensive in the mineral-rich eastern Congo, and warned that the Trump administration will take action against "spoilers" of the deal.
The accord didn't include the rebel group, which is negotiating separately with Congo and agreed earlier this year to a ceasefire that both sides accuse the other of violating.
However, it obliges Rwanda to halt support for armed groups like M23 and work to end hostilities.
US Secretary of State Marco Rubio, decried the situation on X.
"Rwanda's actions in eastern DRC are a clear violation of the Washington Accords signed by President Trump, and the United States will take action to ensure promises made to the President are kept."
The rebels' advance pushed the conflict to the doorstep of neighbouring Burundi, which has maintained troops in eastern Congo for years, heightening fears of a broader regional spillover.
More than 100 armed groups are vying for a foothold in mineral-rich eastern Congo, near the border with Rwanda, most prominently M23.
The conflict has created one of the world's most significant humanitarian crises, with more than seven million people displaced, according to the UN agency for refugees.
Local UN partners report that more than 200,000 people have been displaced across the province since December 2.
Civilians also have crossed into Burundi, and there have been reports of shells falling in the town of Rugombo, on the Burundian side of the border, raising concerns about the conflict spilling over into Burundian territory.
Congo, the US and UN experts accuse Rwanda of backing M23, which has grown from hundreds of members in 2021 to around 6500 fighters.
