Harare (CNN) A prominent critic of President Robert Mugabe is facing 20 years in jail if convicted of multiple counts of subverting Zimbabwe's government.

After posting a series of videos calling on Zimbabweans to demonstrate against fuel shortages and sudden price hikes, Pastor Evan Mawarire was arrested Sunday.

In a court appearance Monday, Mawarire pleaded not guilty to subversion charges launched against him in February. He is scheduled to appear in court again on Tuesday regarding Sunday's charges.

The videos helped ignite the largest protests in several years in the once prosperous southern African country.

On July 12, 2016, Mawarire was first detained by police for "inciting public violence" -- a count that was subsequently amended to a much more serious charge of subversion -- but was released the next day under immense public pressure, when the court ruled the police had violated his rights.

The pastor fled the country, fearing for his life, spending six months in self-imposed exile in the United States. On his return to Harare in February, he was immediately arrested and again charged with subversion.

On Saturday, the US embassy in Harare issued a statement asking Mugabe's government to ensure Mawarire's fair trial in the case that came out of the February arrest.

"The United States is monitoring the trial of Pastor Evan Mawarire, and calls for an end to arbitrary arrests and intimidation for political purposes. We support freedom of expression and the right of peaceful assembly. We call on the Government of Zimbabwe to respect and to protect the human rights of all persons in Zimbabwe, consistent with international human rights norms," said David McGuire, the spokesman for the US Embassy in Harare.

Mugabe has called the pastor a fake, and said he was "foreign-sponsored."

Before his most recent arrest, Mawarire tweeted, "I'm just about to finish preaching and I'm told the police are waiting outside."

I'm just about to finish preaching and I'm told the police are waiting for me outside — #ThisFlag E Mawarire (@PastorEvanLive) September 24, 2017

Inside the church, Mawarire told his congregants: "Zimbabwe, you will be free. There is no bondage or operation that will hold you back. We will not fear anybody. They (police) are outside here. They are waiting. I will not be afraid. I will not be put into fear. I will go outside now and see where this will lead us. I do not know what will happen."