By Jason Hanna, Nicole Chavez and Flora Charner, CNN

(CNN) -- As two declared presidents vie for power in Venezuela, a UN human rights official says at least 20 people have died in protest-related violence this week in the Latin American country, sparking her concern that the situation there "may rapidly spiral out of control."

The 20 people allegedly were shot to death by security forces or members of pro-government armed groups during demonstrations Tuesday and Wednesday, UN human rights High Commissioner Michelle Bachelet said Friday, citing "credible local sources."

This comes amid a mounting crisis inside and outside Venezuela over its future, with opposition leader Juan Guaido having declared himself acting president, openly rebelling against President Nicolas Maduro, whose opponents allege was illegitimately elected for a second term.

More than 350 demonstrators have reportedly been detained during the scores of protests that have occurred in Venezuela this week, Bachelet's office said.

"I am extremely concerned that the situation in Venezuela may rapidly spiral out of control with catastrophic consequences," Bachelet said.

"Any violent incident resulting in death or injury should be subject to an independent and impartial investigation to find out whether there was excessive use of force by the authorities, or if crimes have been committed by members of armed groups, whether pro-government or otherwise," she said.

Guaido, the head of Venezuela's opposition-led legislature, declared himself acting president Wednesday amid massive anti-government protests.

The power struggle has sparked a diplomatic row as well, with the United States and at least a dozen other countries saying they'll recognize Guaido, while Russia, China and others back Maduro.

Maduro, who accuses the United States of meddling, said Thursday he's closing Venezuela's embassy and all its consulates in the United States, and has given US diplomats until this weekend to leave his country.

"I believe there's no doubt that [US President] Donald Trump wants to impose a de facto government, unconstitutional government, a coup against the people and democracy in Venezuela," Maduro said Thursday in a lengthy speech that aired live on state broadcaster VTV. "There's no doubt that is Donald Trump with his craziness of believing he's the world's police."

Guaido wants diplomats to stay in US

Speaking at Venezuela's Supreme Court on Thursday, Maduro said Venezuela's diplomats and consulate personnel in the US will return to the Latin America country by Saturday.

Maduro reiterated his decision to cut diplomatic ties with the US and continuously slammed President Donald Trump, among other top US officials.

All non-emergency US employees were ordered to leave Venezuela on Thursday, according to a security alert. US Secretary of State Mike Pompeo had earlier brushed off Maduro's order that US diplomats leave the country, saying the US doesn't recognize his power to do so.

The US Embassy in Caracas opened Thursday but all visa appointments were canceled.

Late Thursday night, Guaido called on Venezuelan diplomats to stay in the US.

"You have in your hands the decision to stay and be recognized by the people of Venezuela and by the one who is writing you today [himself]," Guaido said in a series of tweets, adding that diplomatic staff have "legitimate, internationally recognized authority."

He said he and his cabinet "will be working on protecting the goods of the nation" as they have been "stolen" by the current regime.

"We have to remember that we are still in a dictatorship in Venezuela and that they are still withholding the weapons and that represents a threat," Guaido said in an interview with Univision television network.

Guaido also said he would consider amnesty for Maduro and his allies if they don't hamper Guaido's ascension to temporary power. He has called for elections.

Venezuela military loyal to Maduro

Top military brass still appear to back Maduro. Hours before his speech Thursday, military commanders and the country's defense minister expressed allegiance to him.

In military dress uniform and flanked by members of the military's high command, Defense Minister Vladimir Padrino said that Maduro was the country's legitimate leader -- and that Washington and other regional powers were trying to "knock out progressive governments that make them uncomfortable."

"We will not bend to foreign intervention or a government not elected by the people," Padrino said in an address aired by state broadcaster VTV.

Moments before Padrino spoke, VTV aired messages of support from Venezuela's eight regional commanders from their military outposts throughout the country.

US calls for Security Council meeting

Pompeo urged all members of the regional Organization of American States to support Guaido, saying Maduro's regime is "now defunct" and illegitimate.

Pompeo announced that the United States, in response to a request from the opposition-led legislature, is ready to give Venezuela $20 million in aid to help with food and medicine shortages, to be distributed "as soon as logistically possible."

The United States said it has asked for a Saturday open meeting of the UN Security Council to discuss Venezuela.

Trump vowed in a statement Wednesday to use "the full weight of United States economic and diplomatic power to press for the restoration of Venezuelan democracy."

Depending on Maduro's response to the protests, Trump is prepared to take a range of actions in retaliation, including possible oil sanctions, two sources familiar with White House deliberations said.

Countries divided over leadership crisis

While many world leaders quickly expressed their support for Guaido, many others stood behind Maduro.

Describing Maduro's regime as illegitimate, several countries, including the United States, Argentina, Brazil, Canada, Chile, Colombia, Ecuador, Paraguay and Peru, recognized Guaido as president.

British Foreign Secretary Jeremy Hunt said the UK believes "Juan Guaido is the right person to take Venezuela forward."

European Council President Donald Tusk expressed support for him and called on other European countries to join him.

Russia, China, Cuba and Turkey are among the nations backing Maduro -- and Russia in particular is criticizing the United States for undermining him.

Washington's interference in Venezuela could lead to "lawlessness and bloodshed," the Russian Foreign Ministry said in a statement.

"We see in the unceremonious actions of Washington a new demonstration of total disregard for the norms and principles of international law, an attempt to play the role of the self-proclaimed arbiter of the destinies of other nations," the statement reads.

A few other countries like Mexico have said they prefer to stay on the sidelines. Mexico's Foreign Minister Marcelo Ebrard said Thursday the country will follow its policy of nonintervention.

UN Secretary-General Antonio Guterres called for dialogue.

Speaking at the World Economic Forum in Davos, Switzerland, the UN chief said he hoped discussions could "avoid an escalation that could lead to the kind of conflict that would be a total disaster for Venezuela and the Venezuelan people and for the region."

This is not the first time that the US and many of Venezuela's neighbors declare they wouldn't recognize Maduro's presidency. Maduro's election in May and the country's deep economic crisis was a flashpoint for many of them.

The election was boycotted by opposition groups and largely discredited by opponents, with hundreds of complaints of election violations and a low turnout.

This story has been updated to correct the spelling of Juan Guaido's last name.