United Kingdom Prime Minister Boris Johnson shared details of the phone call he had with Biden Saturday, as the president continues to reach out to U.S. allies and partners in the days after his inauguration.

In a post on social media, Johnson said it was "great to speak" with Biden. "I look forward to deepening the longstanding alliance between our two countries as we drive a green and sustainable recovery from COVID-19," he added, along with a photo of himself smiling on the phone. 

During the call, Johnson "warmly welcomed" Biden's decision to rejoin the Paris Climate Agreement and the World Health Organization, a Downing Street spokesperson said.  

"They also discussed the benefits of a potential free trade deal between our two countries, and the Prime Minister reiterated his intention to resolve existing trade issues as soon as possible," the spokesperson said.

President Biden has been making his first calls to foreign leaders as president. On Friday, he spoke with Canadian Prime Minister Justin Trudeau and Mexican President Andrés Manuel López Obrador.

In previewing the president's early calls to foreign heads of state, White House Press Secretary Jen Psaki had said the president would first prioritize close "partners and allies," because the president "feels it's important to rebuild those relationships and to address the challenges and threats we're facing in the world." 

-ABC News' Rashid Haddou, Jordyn Phelps and Molly Nagle

While Wednesday's Inauguration Day was steeped in tradition, it held many firsts too.

During the actual ceremony, Harris was sworn in as both the first female and person of color vice president, Amanda Gorman was the youngest inaugural poet in history, and Andrea Hall recited the Pledge of Allegiance in sign language.

But even as the official ceremony ended, the day of firsts didn't. Hours after the inaugural ceremony, Harris administered the oath of office to Rev. Raphael Warnock, the first Black senator from Georgia, and Jon Ossoff, the first Jewish senator from the South since the 1880s. Also, the White House website revised its contact form by adding gender-inclusive pronoun and prefix options including "they/them" and the gender-neutral prefix of "Mx."

-ABC News' Kiara Brantley-Jones and Robert Zepeda

Inauguration Day was historic, with Kamala Harris becoming the first woman and person of color to become vice president. But if all of Biden's cabinet nominations are confirmed, Harris wouldn't be the only one making history.

Twelve of Biden's nominations for Cabinet and Cabinet-level positions are women, including eight women of color. If they're all confirmed, it would shatter former President Bill Clinton's record of nine women serving concurrently, which happened during his second term.

Janet Yellen, who was approved unanimously in the Senate Finance Committee on Friday, is nominated to be the first female secretary of the U.S. Treasury. Her confirmation vote is expected to take place early next week.

-ABC News' Deena Zaru

The U.S. Department of State "strongly" condemned the mass arrests in Russia of protesters in a statement Saturday.

The department called for the release of the protesters and Kremlin critic Alexey Navalny, who was jailed last week after he returned to the country for the first time since recovering from poisoning with a nerve agent.

"The United States strongly condemns the use of harsh tactics against protesters and journalists this weekend in cities throughout Russia," State Department spokesperson Ned Price said in the statement. "The United States will stand shoulder-to-shoulder with our allies and partners in defense of human rights -- whether in Russia or wherever they come under threat."

Tens of thousands of people joined protests across dozens of cities in Russia Saturday. By early evening, police had detained over 1,600 people, according to OVD-Info, a group that monitors arrests.

In its statement, the State Department criticized the growing state of repression in Russia, from harassing protesters to threatening social media platforms, and defended Russians’ rights to protest and to free and fair elections.

It also called on Russia to explain the use of a chemical weapon on its soil and to cooperate with an international investigation.

-ABC News' Connor Finnegan