Google on Monday opened its Project Fi mobile phone service to anyone in the United States using its latest model Nexus smartphones.

The public launch of Project Fi marked the end of a 10-month, invitation-only test phase and comes as the Internet giant's first foray in being a mobile phone service provider.

"We're excited to be exiting our invitation-only mode and opening up Project Fi so that people across the US can now sign up for service without having to wait in-line for an invite," product manager Simon Arscott said in a blog post.

"With Project Fi, we deliver fast wireless service with the flexibility to use it where you want—even internationally—and a monthly bill that's simple and easy to understand."

Project Fi is only available for Nexus 6P, Nexus 5X, and Nexus 6 smartphones from Google and work with a special SIM card that supports connections to multiple cellular networks.

Project Fi enabled smartphones can link to one of two 4G LTE networks or to Wi-Fi, automatically selecting optimal connections and switching from one to another without interrupting service, according to Google.

"As you go about your day, Project Fi automatically connects you to more than a million free, open Wi-Fi hotspots we've verified as fast and reliable," Google said in a blog post.

"Once you're connected, we help secure your data through encryption."

Basic Project Fi plans cost $20 monthly for unlimited domestic voice calls as well as unlimited domestic and international text messaging. Fi boasts cellular coverage in more than 120 countries.

Data service costs $10 per gigabyte, with people only charged for what they use.

Google promoted Fi service with discounted prices on Nexus 5X smartphones.

The move by Google is seen as a way to lure consumers away from Apple and promote Google-branded Android phones with an a low-cost mobile plan.