US border patrol agents discovered a new hidden tunnel running from Mexico that led to the arrest of 30 illegal immigrants.

Around 1.30am on Saturday, agents patrolling at the Otay Mesa Port of Entry in San Diego, California, saw a group of several dozen people and tried to approach them for questioning.

Some people in the group ran back into what the agents discovered to be the exit of a cross-border tunnel with a ladder inside near Drucker Lane and Siempre Viva Road.

Agents made 30 arrests both inside and outside the tunnel. Of those arrested, 23 were Chinese nationals and seven were Mexican nationals.

US border patrol agents have arrested 30 illegal immigrants after they tried to cross into the country through a newly discovered hidden tunnel leading from Mexico (pictured)

According to investigators, the tunnel began in a building in the Garita de Otay area in Tijuana, approximately 328 feet south of the US-Mexico border in San Diego (Pictured, Otay Mesa port of entry)

According to investigators, the tunnel began in a building in the Garita de Otay area in Tijuana, approximately 328 feet south of the US-Mexico border in San Diego and three miles from the border crossing bridge.

The opening to the tunnel had been covered by dry bush and branches.

Mexican authorities are working to determine who is responsible for the build-out and operation of the smuggling tunnel.

Authorities say this is the 13th underground passageway discovered along California's border with Mexico since 2006.

The use of such tunnels is not new, but they have typically been used for drug smuggling.

As of 4pm on Saturday, US Customs and Border Protection (CBP) agent Eduardo Olmos told NBC 7 that no drugs had been discovered inside the tunnel.

The use of such tunnels (route highlighted in red) is not new, but they have typically been used for drug smuggling. As of 4pm on Saturday, no drugs had been discovered inside the tunnel

'Preliminarily it appears this latest tunnel may be an extension of an incomplete tunnel previously discovered and seized by Mexican authorities,' the CBP wrote in a press release.

'While subterranean tunnels are not a new occurrence along the California-Mexico border, they are more commonly utilized by transnational criminal organizations to smuggle narcotics.

'However, as this case demonstrates, law enforcement has also identified instances where such tunnels were used to facilitate human smuggling.'