No space. For example, if you want to dial a Beijing number, you enter "00861012345678", where 0086 is the code of China, 10 is the code of Beijing, 12345678 is the phone number. On some cellphones or devices, you enter +86 instead.
If you want to dial a Hong Kong number, you enter 0085212345678, or + 85212345678, where 00852 or +852 is the code of Hong Kong, 12345678 is the phone number.
Also, for the cell phone numbers in mainland China, you don't need the code of city, you only need to enter the code of china and the cellphone number.