They say Qatar backs terrorist groups including Islamic State (IS).
The Saudi state news agency SPA said Riyadh had closed its borders, severing land, sea and air contact with Qatar and largely isolating it.
It cited officials as saying it was to "protect its national security from the dangers of terrorism and extremism".
The unprecedented move is being seen as the most serious split yet between powerful Gulf countries, who are also close US allies.
The row comes two weeks after the same four countries blocked Qatari news sites. Controversial comments by Qatari emir Sheikh Tamim bin Hamad al-Thani, appearing to criticise Saudi Arabia,
The government in Doha dismissed the comments as fake, attributing the report to a "shameful cybercrime".
In the latest developments:
- The United Arab Emirates has given Qatari diplomats 48 hours to leave the country. Abu Dhabi accuses Doha of "supporting, funding and embracing terrorism, extremism and sectarian organisations," state news agency WAM said
- The UAE state airline Etihad Airways said it would suspend all flights to and from Doha from 02:45 local time on Tuesday
- Bahrain's state news agency said the country was cutting ties with Qatar because Doha was "shaking the security and stability of Bahrain and meddling in its affairs"
- US Secretary of State Rex Tillerson, speaking in Sydney, urged the countries to resolve their differences through dialogue.
The Saudi-led Arab coalition fighting Yemen's Houthi rebels also expelled Qatar from its alliance because of Doha's "practices that strengthen terrorism" and its support to groups "including al-Qaeda and Daesh [also known as IS], as well as dealing with the rebel militias", according to SPA.
Qatar has provided its warplanes to carry out air strikes against the Houthi rebels.
Qatar, which is due to host the football World Cup in 2022, has so far made no public comments on the latest developments.
SOURCE : BBC