Thousands of daytrippers jostled for selfie space and elbowed their way to the sea at a popular north Vietnam beach over the weekend, with extra vigilant lifeguards watching worriedly over the summer surge.

There are few beaches in this part of the country, and Sam Son in Thanh Hoa province has long been a go-to destination with its white sand coastline and blue waters.

But its beauty has proven both a blessing and a curse, leading vacationers to pack out the 16 kilometre-long (9.9 mile-long) beach.

"Today, there are too many tourists," said Le Huu Mui, an 80-year-old visiting with family.

He last came to Sam Son five years ago and was shocked at the difference.

"We have to hustle on the beach and it is less comfortable than previous years."

On Saturday couples snapped selfies in front of elaborate sandcastles and families played spirited games of tug of war.

But not everyone was relaxing.

"We have to keep an eye on everything," said lifeguard Nguyen Huu Linh, 41. "There are cases of tourists not listening to us."

Visitor Dao Quyet Tien, who comes to the beach frequently, has also noticed murkier waters as more swimmers stir up sand and sediment.

"There have been a few times...when the beach water was dirty," he told AFP. "It's not comfortable to swim." 

But he admitted the crowds also brought a lively atmosphere to Sam Son.

"With the beach busy like this, I can meet new people -- that's what I like about about it," the 27-year-old said. 

© AFP