Bangladesh's sprawling capital inaugurated its first ever metro rail line on Wednesday as authorities work to ease congestion that has throttled growth and inflamed tempers in the gridlocked metropolis.

Dhaka is one of the world's most densely populated cities, and daily commutes along its car-clogged roads are a source of constant frustration for its 22 million people.

Local researchers say the capital's economy loses upwards of $3 billion each year in lost work time due to traffic jams, often worsened by regular street protests and monsoonal downpours.

The new elevated train network has been in development for nearly a decade and is slated to grow to over a hundred stations and six lines criss-crossing the city by 2030.

Wednesday saw the start of operations on a section of the first line connecting a neighbourhood on Dhaka's periphery with the city centre, built with a $2.8 billion price tag and largely funded by Japanese development funds.

"This metro rail is also another matter of pride for us," said Prime Minister Sheikh Hasina at a ceremony to mark the first service.

"We promised to eradicate traffic jams from Dhaka," she added. "With the six metro rail lines, we will be able to do so."

The line is expected to carry 60,000 people each hour when it is fully operational and its opening has been keenly anticipated by commuters.

"We are counting on it. It will reduce public suffering," Mostafizur Rahman, who spends nearly three hours riding a bus to work each morning, told AFP.

Hasina used the opening ceremony to commemorate six Japanese rail engineers working on the project who were killed during an attack on a Dhaka cafe by Islamist extremists in 2016.