Electors in the North Kivu provincial capital of Goma lined up, some of them for hours, to obtain their voting card ahead of the December ballot, AFP reporters saw.
"The army and police have been mobilized to secure the electoral process," the province's military governor, Lieutenant-General Constant Ndima, told reporters.
"All displaced people will be registered according to their places of origin," he said.
The vast central African country has a long history of political turbulence and disputed elections.
Its last presidential vote, in December 2018, saw the nation's first peaceful handover of power, with the election of former opposition leader Felix Tshisekedi, although the results were bitterly contested by his rivals.
Since then, the country's volatile east has been struggling with the rise of armed groups, including the Allied Democratic Forces (ADF) -- which the Islamic State group describes as its regional affiliate -- and the M23, which the government says is supported by neighboring Rwanda.
Voter registration in western Democratic Republic of Congo began last December and was followed last month in the center and southeast of the country.
The third and final phase comprises registration in seven other provinces, three of which - North Kivu, South Kivu and Ituri - have been badly hit by armed groups.
Ndima said that areas that were under the control of the M23, which controls large areas north and northwest of Goma, would undergo voter registration after they have been "liberated."
The December 20 presidential elections will take place in tandem with ballots for national legislators, provincial deputy and local councillors.
DRC Starts Eastern Province Voter Registration
FILE: Voters queue to cast their ballots at a symbolic polling station on December 30, 2018, at Malepe Stadium in Beni, North Kivu, DRC.
Voter registration ahead of year-end presidential elections got underway on Thursday in DRC's troubled east, where armed groups control swathes of territory and hundreds of thousands have fled their homes.
GOMA —