REAL ID requirements for those flying within the United States began Wednesday after nearly 20 years of delays.

The day ahead of the deadline, people lined up at government offices across the country to secure their compliant IDs.

Homeland Security Secretary Kristi Noem on Tuesday assured people who don't yet have a REAL ID but need to take a domestic flight on Wednesday that they will be able to fly after clearing additional identity checks.

Some complained about the need to secure the ID after waiting in line for hours.

The Transportation Security Administration warned people who don’t have identification that complies with REAL ID requirements to arrive early at the airport and be prepared for advanced screening to avoid causing delays.

Noem told a congressional panel that 81% of travelers already have REAL IDs. She said security checkpoints will also accept passports and tribal identification, as they have been doing.

Those who still lack an identification that complies with the REAL ID law “may be diverted to a different line, have an extra step,” Noem said.

“But people will be allowed to fly,” she said. “We will make sure it’s as seamless as possible.”

REAL ID is a federally compliant state-issued license or identification card that Homeland Security says is a more secure form of identification. It was a recommendation by the 9/11 Commission and signed into law in 2005. It was supposed to be rolled out in 2008, but the implementation had been repeatedly delayed.

¨The whole idea here is to better validate those individuals that were encountering a checkpoint to ensure they are who exactly they say they are,” said Thomas Carter, TSA’s Federal Security Director in New Jersey.

Carter said those without a REAL ID should give themselves extra time to clear security.

“If they do that, I do not believe that this will cause people to miss their flights if they take that additional time in,” he said.

Besides serving as a valid form of identification to fly domestically, people will also need a REAL ID to access certain federal buildings and facilities.

State government offices that issue driver's licenses and state IDs have seen a significant increase in demand for REAL ID, and some have extended their office hours to meet the demand. Some officials have recommended people wait for a while to get REAL ID-compliant licenses and cards if they don't have a flight planned in the next few months.