How do I escape ampersands in batch files?

From a cmd:

• & is escaped like this: ^& (based on @Wael Dalloul's answer)
• % does not need to be escaped

An example:

start http://www.google.com/search?client=opera^&rls=en^&q=escape+ampersand%20and%20percentage+in+cmd^&sourceid=opera^&ie=utf-8^&oe=utf-8

From a batch file

• & is escaped like this: ^& (based on @Wael Dalloul's answer)
• % is escaped like this: %% (based on the OPs update)

An example:

start http://www.google.com/search?client=opera^&rls=en^&q=escape+ampersand%%20and%%20percentage+in+batch+file^&sourceid=opera^&ie=utf-8^&oe=utf-8

& is used to separate commands. Therefore you can use ^ to escape the &.