@RemoteAction
global static Object invoke(String methodName, String[] params) {

    // Create a Continuation for the HTTPRequest
    Continuation con = new Continuation(60);
    con.continuationMethod = 'callback';

    ...
    //Add http request to continuation

    return con;
}

global static Object callback(list<string> labels, Object state) {
    system.debug(labels);
    for (string l : labels) {
        system.debug(l);
        HttpResponse response = Continuation.getResponse(l);
        system.debug(response);
    }

    ...
}