WASHINGTON — While Mayor Muriel E. Bowser of Washington, D.C., was grappling with the riot that tore through the Capitol last week, another crisis was slowly unfolding: a surge of coronavirus in the district.

Washington averaged 290 new coronavirus cases a day in the seven-day period that ended Sunday, the most the city has seen during any week of the pandemic. The surge is part of a broader upward tide throughout the nation’s Mid-Atlantic region: Virginia, Maryland and Delaware also set weekly case records on Sunday.

On Monday, Ms. Bowser urged vigilance against the virus, noting that the district’s hospitals were strained by coronavirus patients and the rate of positive test results was high.

“We remain concerned — as the rest of the country remains concerned — about the increase in cases,” she said.

Public health experts warned that the district could be headed for an alarming wave of coronavirus infections in the next few weeks, following the riot in the Capitol, when lawmakers and staff members huddled in small rooms to hide from attackers.

“The district is operating under a double whammy,” said Sara Rosenbaum, a professor at the Milken Institute School of Public Health at George Washington University.

The district has the challenges of any large urban center, she noted, including concentrated poverty and large numbers of vulnerable residents, making fighting the virus very difficult.

“On top of that, the district has this unique risk factor, which is exposure to thousands of people who poured into town, maskless, packed into very concentrated areas, and simply ignoring every public health safety precaution,” Ms. Rosenbaum said. “We won’t know for the next couple of days whether we have a further skyrocketing of cases, but it had all the attributes of what we have all come to understand is a superspreader event — which is huge numbers of people packed together indoors, screaming.”

Washington’s latest surge predates last week’s riot, and not enough time has elapsed for the scale of infections that occurred that day to show up in the district’s data. The full impact may never be known: Infections are generally counted where a person lives, so out-of-town residents who caught the virus during the unrest would not be included in Washington’s case count.

Representative Bonnie Watson Coleman, Democrat of New Jersey, suggested on Monday that she was one of them.

“Following the events of Wednesday, including sheltering with several colleagues who refused to wear masks, I decided to take a Covid test,” she wrote on Twitter. “I have tested positive.”

Representative Chuck Fleischmann of Tennessee also has announced that he has tested positive after being exposed to his roommate, Representative Gus Bilirakis of Florida. Both lawmakers are Republicans.

Mr. Fleischmann told a Chattanooga TV station, WRCB, that he was sent a notification on Wednesday that his roommate had tested positive, but did not receive it right away because he was locked down in a secure location during the riot. He said he did not know how many other lawmakers he had come in contact with that day.

After enduring a spring surge of the coronavirus, Washington kept its case numbers relatively low through the summer and into the fall. On Oct. 1, the district, with about 700,000 residents, was averaging fewer than 40 new cases a day.

The outlook has worsened since then. Case numbers rose steeply for two months and peaked in early December. They ebbed a bit in the days before Christmas, but that progress was undone in the first days of 2021.

Around the city, the streets continue to bustle with traffic, but everything else is eerily quiet. Museums and theaters are shuttered, restaurants closed except to outdoor diners or for takeout, downtown sidewalks all but empty. More than half the city’s residents have taken a Covid-19 test. Depending on the time of day, the lines of people waiting for free tests at firehouses and community centers can stretch for blocks, and hours.

Ms. Bowser asked the federal government on Saturday to declare a “pre-emergency” situation for the District of Columbia, citing not only last week’s riot at the Capitol but the increasingly rapid spread of the coronavirus.

Early in the pandemic, bars in the city were jammed with patrons flouting mask requirements, but now, caution is the rule. A check by health officials last month found that 72 percent of people were wearing masks correctly. Hospitals are not yet full; about six out of seven beds are filled.

One bright spot: The city’s vaccination program is gaining steam. On Monday, it began scheduling shots for anyone 65 or older.

Many residents remain wary of the rising threat from the virus. Even after so many months of the pandemic, much feels uncertain. Drew Schneider, a community blogger in Petworth, a mixed-income neighborhood in northwest Washington, said the virus had rampaged through his sister’s family, sickening her, her husband and their two children. Their symptoms varied wildly: One felt fine. Another was ill for weeks. A third recovered after a couple of days but is plagued with headaches. The fourth had gastrointestinal problems.

Their experience was scary, Mr. Schneider said, and the accelerating spread of the virus had him increasingly concerned.

“You just never know what’s going to happen,” he said. “You have to keep your guard up, keep your mask on and wait for the vaccine.”

Julie Wineinger runs Lulabelle’s, an ice cream and coffee shop, and Willow, a women’s fashion store, in a commercial block dotted with vacant storefronts. When the pandemic hit in March, Ms. Wineinger added grocery staples like bread to her stock, and allowed customers to order online. Her clothing shop has closed, except for online orders.

“I don’t necessarily say we’re going gangbusters, but it’s enabling us to stay open and pay employees,” she said.

Even so, the last few months have been getting harder. Business has “definitely gone down,” she said.

The pandemic has hit her family, too. Recently, Ms. Wineinger’s sister caught the virus and recovered. But her grandmother, who got sick last year, died of Covid-19 in November.

Michael Wines reported from Washington, and Julie Bosman from Chicago. Mitch Smith and Dave Philipps contributed reporting.