diff --git a/compose/project.py b/compose/project.py
index c02a1d8dd..af6b9ce0c 100644
--- a/compose/project.py
+++ b/compose/project.py
@@ -276,19 +276,31 @@ class Project(object):
 
     def get_service_scale(self, service_dict):
         # service.scale for v2 and deploy.replicas for v3
-        scale = service_dict.get('scale', 1)
+        scale = service_dict.get('scale', None)
         deploy_dict = service_dict.get('deploy', None)
-        if deploy_dict:
-            if deploy_dict.get('mode', 'replicated') == 'replicated':
-                scale = deploy_dict.get('replicas', scale)
-            # deploy may contain placement constraints introduced in v3.8
-            max_replicas = deploy_dict.get('placement', {}).get(
-                'max_replicas_per_node',
-                scale)
-            scale = min(scale, max_replicas)
-            if max_replicas < scale:
-                log.warning("Scale is limited to {} ('max_replicas_per_node' field).".format(
-                    max_replicas))
+        if not deploy_dict:
+            return 1 if scale is None else scale
+
+        if deploy_dict.get('mode', 'replicated') != 'replicated':
+            return 1 if scale is None else scale
+
+        replicas = deploy_dict.get('replicas', None)
+        if scale and replicas:
+            raise ConfigurationError(
+                "Both service.scale and service.deploy.replicas are set."
+                " Only one of them must be set."
+            )
+        if replicas:
+            scale = replicas
+        # deploy may contain placement constraints introduced in v3.8
+        max_replicas = deploy_dict.get('placement', {}).get(
+            'max_replicas_per_node',
+            scale)
+
+        scale = min(scale, max_replicas)
+        if max_replicas < scale:
+            log.warning("Scale is limited to {} ('max_replicas_per_node' field).".format(
+                max_replicas))
         return scale
 
     def start(self, service_names=None, **options):